这是我的代码

#include <bits/stdc++.h>
using namespace std;  
struct Cow {  
    int row, col;  
    bool operator<(const Cow& other) const {  
        return row < other.row || (row == other.row && col < other.col);  
    }  
};  
  
int main() {  
    int r, c;  
    cin >> r >> c;  
    int n;  
    cin >> n;  
    vector<Cow> cows;  
    for (int i = 0; i < n; i++) {  
        int row, col;  
        cin >> row >> col;  
        cows.push_back({row, col});  
    }  
    sort(cows.begin(), cows.end());  
    set<pair<int, int>> visited;  
    for (const auto& cow : cows) {  
        visited.insert({cow.row, cow.col});  
        if (is_sorted(visited.begin(), visited.end(), [](const pair<int, int>& a, const pair<int, int>& b) {  
                return a.first < b.first || (a.first == b.first && a.second < b.second);  
            })) {  
            cout << 1 << endl;  
            for (const auto& v : visited) {  
                if (v == make_pair(cow.row, cow.col)) {  
                    cout << i + 1 << endl;  
                    break;  
                }  
            }  
            return 0;  
        }  
        visited.erase(cow);  
    }  
    cout << -1 << endl;  
    return 0;  
}

请问为什么会编译失败，以下是编译失败的程序

/tmp/compiler_4rteggt4/src: 在函数‘int main()’中:
/tmp/compiler_4rteggt4/src:31:29: 错误：‘i’在此作用域中尚未声明
   31 |                     cout << i + 1 << endl;
      |                             ^
/tmp/compiler_4rteggt4/src:37:26: 错误：no matching function for call to ‘std::set<std::pair<int, int> >::erase(const Cow&)’
   37 |         visited.erase(cow);
      |                          ^
In file included from /nix/store/496xlqhx1mk41sdmrl58xi5y4pa0alys-luogu-gcc-9.3.0/include/c++/9.3.0/set:61,
                 from /nix/store/496xlqhx1mk41sdmrl58xi5y4pa0alys-luogu-gcc-9.3.0/include/c++/9.3.0/x86_64-unknown-linux-gnu/bits/stdc++.h:87,
                 from /tmp/compiler_4rteggt4/src:1:
/nix/store/496xlqhx1mk41sdmrl58xi5y4pa0alys-luogu-gcc-9.3.0/include/c++/9.3.0/bits/stl_set.h:654:7: 附注：candidate: ‘std::set<_Key, _Compare, _Alloc>::iterator std::set<_Key, _Compare, _Alloc>::erase(std::set<_Key, _Compare, _Alloc>::const_iterator) [with _Key = std::pair<int, int>; _Compare = std::less<std::pair<int, int> >; _Alloc = std::allocator<std::pair<int, int> >; std::set<_Key, _Compare, _Alloc>::iterator = std::_Rb_tree_const_iterator<std::pair<int, int> >; std::set<_Key, _Compare, _Alloc>::const_iterator = std::_Rb_tree_const_iterator<std::pair<int, int> >]’
  654 |       erase(const_iterator __position)
      |       ^~~~~
/nix/store/496xlqhx1mk41sdmrl58xi5y4pa0alys-luogu-gcc-9.3.0/include/c++/9.3.0/bits/stl_set.h:654:28: 附注：  no known conversion for argument 1 from ‘const Cow’ to ‘std::set<std::pair<int, int> >::const_iterator’ {aka ‘std::_Rb_tree_const_iterator<std::pair<int, int> >’}
  654 |       erase(const_iterator __position)
      |             ~~~~~~~~~~~~~~~^~~~~~~~~~
/nix/store/496xlqhx1mk41sdmrl58xi5y4pa0alys-luogu-gcc-9.3.0/include/c++/9.3.0/bits/stl_set.h:684:7: 附注：candidate: ‘std::set<_Key, _Compare, _Alloc>::size_type std::set<_Key, _Compare, _Alloc>::erase(const key_type&) [with _Key = std::pair<int, int>; _Compare = std::less<std::pair<int, int> >; _Alloc = std::allocator<std::pair<int, int> >; std::set<_Key, _Compare, _Alloc>::size_type = long unsigned int; std::set<_Key, _Compare, _Alloc>::key_type = std::pair<int, int>]’
  684 |       erase(const key_type& __x)
      |       ^~~~~
/nix/store/496xlqhx1mk41sdmrl58xi5y4pa0alys-luogu-gcc-9.3.0/include/c++/9.3.0/bits/stl_set.h:684:29: 附注：  no known conversion for argument 1 from ‘const Cow’ to ‘const key_type&’ {aka ‘const std::pair<int, int>&’}
  684 |       erase(const key_type& __x)
      |             ~~~~~~~~~~~~~~~~^~~
/nix/store/496xlqhx1mk41sdmrl58xi5y4pa0alys-luogu-gcc-9.3.0/include/c++/9.3.0/bits/stl_set.h:706:7: 附注：candidate: ‘std::set<_Key, _Compare, _Alloc>::iterator std::set<_Key, _Compare, _Alloc>::erase(std::set<_Key, _Compare, _Alloc>::const_iterator, std::set<_Key, _Compare, _Alloc>::const_iterator) [with _Key = std::pair<int, int>; _Compare = std::less<std::pair<int, int> >; _Alloc = std::allocator<std::pair<int, int> >; std::set<_Key, _Compare, _Alloc>::iterator = std::_Rb_tree_const_iterator<std::pair<int, int> >; std::set<_Key, _Compare, _Alloc>::const_iterator = std::_Rb_tree_const_iterator<std::pair<int, int> >]’
  706 |       erase(const_iterator __first, const_iterator __last)
      |       ^~~~~
/nix/store/496xlqhx1mk41sdmrl58xi5y4pa0alys-luogu-gcc-9.3.0/include/c++/9.3.0/bits/stl_set.h:706:7: 附注： 备选需要 2 实参，但提供了 1 个