求助第四个点WA

#include <iostream>
#include <cmath>
#include <cstdio>
#include <string>
#include <vector>
#include <queue>
#include <map>
using namespace std;
long long n, m;
long long a[100005];
long long check(long long k) {
    long long op = 0;
    long long cnt = 0;
    for (int i = 1; i <= n; i++) {
        if ((cnt + a[i]) > k) {
            op++;
            cnt = a[i];
        }
        else
            cnt += a[i];
    }
    return op >= m;
}
int main() {

    cin >> n >> m;
    
    for (int i = 1; i <= n; i++) {
        cin >> a[i];
    }
    long long l = 1, r = 4e18;
    while (l < r) {
        long long mid = l + r >> 1;
        if (check(mid)) {
            l = mid + 1;
        }
        else r = mid;
    }
    cout << l ;
    return 0;
}