思路

#include <bits/stdc++.h>
using namespace std;

#define ll long long

const int N = 100010;

int n, k, T;
int a[N], b[N];
bool check(int x)
{
	for (int i = 1; i <= x; i++) b[i] = a[i];
	sort(b + 1, b + 1 + x);
	ll sum = 0, v = 0;
	for (int i = 1; i < k; i++)
	{
		sum += (ll)b[i] * b[i];
		v += b[i];
	}
	for (int i = k; i <= x; i++)
	{
		sum += (ll)b[i] * b[i] - (ll)b[i - k] * b[i - k];
		v += b[i] - b[i - k];
		double t = 1.0 * v / k;
		if ((sum - 1.0 * v * v / k) / k < T)
		{
			//cout<< (sum - 1.0 * v * v / k) / k << endl;
			return true;
		}
	}
	return false;
}
int main()
{
	scanf("%d%d%d", &n, &k, &T);
	for (int i = 1; i <= n; i++) scanf("%d", &a[i]);
	int l = k, r = n, mid;
	while (l < r)
	{
		mid = (l + r) >> 1;
		if (check(mid)) r = mid;
		else l = mid + 1;
	}
	if (r == n && !check(n)) puts("-1");
	else printf("%d", r);
	return 0;
}