这么写

#include<iostream>
#define int long long
using namespace std;

int n,k;
int nums[100005];

int calc(){
	int ans = 0;
	for(int i = 0;i < n;i ++){
		ans += nums[i];
	}
	return ans;
}

signed main(){
	cin>>n>>k;
	for(int i = 0;i < n;i ++){
		cin>>nums[i];
		if(i == k - 1){
			nums[i] = - nums[i];
		}
	}
	cout<<calc();
	return 0;
}

不行

这么写

#include<iostream>
#define ll long long
using namespace std;

ll sum;
int n,k;
ll x;
ll nums[100005];

int main(){
	cin>>n>>k;
	for(int i = 0;i < n;i ++){
		cin>>x;
		if(i == k - 1){
			sum -= x;
		}else	sum += x;
	}
	cout<<sum;
	return 0;
}

就可以