只有写成①或添加②才能AC，有人知道这是为什么吗

#include <bits/stdc++.h>
#define int long long
using namespace std;
const int N = 200010;
struct Point {
    int x, y;

    // 向量
    Point operator-(const Point& b)const {
        return { x - b.x,y - b.y };
    }

    // 叉积
    int operator^(const Point& b)const {
        return x * b.y - y * b.x;
    }
}a[N];

// 逆时针
int area(Point a, Point b, Point c) {
    return abs((b - a) ^ (c - a));
}

void solve() {
    int n, k;
    cin >> n >> k;
    for (int i = 1; i <= n; i++) cin >> a[i].x >> a[i].y;
    for (int i = 1; i <= n; i++) a[n + i] = a[i];
    int mid = 1 + k + 1;
    int l = 1, r = 1 + k;
    int res = 0, ma = 0;
    for (int i = 2; i <= r; i++) res += area(a[1], a[i - 1], a[i]);
    for (; l <= n; l++, r++) {
        while (mid < n + l - 1 && area(a[l], a[r], a[mid]) < area(a[l], a[r], a[mid + 1])) mid++;
        // while (mid < n + l - 1 && area(a[l], a[r], a[mid]) <= area(a[l], a[r], a[mid + 1])) mid++; ①
        // if (mid == r) mid++;   ②
        ma = max(ma, res + area(a[l], a[r], a[mid]));
        res -= area(a[l], a[l + 1], a[r]);
        res += area(a[l + 1], a[r], a[r + 1]);
    }
    printf("%.12lf\n", ma * 1.0 / 2.0);
}

signed main() {
    ios::sync_with_stdio(false);
    cin.tie(0);
    int T = 1;
    cin >> T;
    while (T--) {
        solve();
    }
    return 0;
}