UPC10544: 凉宫春日的叹息

#include "bits/stdc++.h"

using namespace std;

typedef long long ll;
const int maxn = 1e6 + 1000;
ll a[maxn];
int n, k;

ll check(ll mid) {
    int j = 0;
    ll res = 0;
    for (int i = 1; i <= n; i++) {
        while (a[i] - a[j] > mid) j++;
        res += i - j;
    }
    return res;
}

int main() {
    //freopen("input.txt", "r", stdin);
    scanf("%d %d", &n, &k);
    for (int i = 1; i <= n; i++) {
        scanf("%lld", &a[i]);
        a[i] += a[i - 1];
    }
    ll l = 0, r = a[n], mid;
    while (l < r) {
        mid = (l + r) >> 1;
        if (check(mid) >= k) r = mid;
        else l = mid + 1;
    }
    printf("%lld\n", r);
    return 0;
}