因为是大数吗，呵呵，greater也一样

我悟了，应改为

#include <bits/stdc++.h>
using namespace std;
#define ll long long
ll a[1010];
int main() {
	ll n,k;
	cin >> n>>k;
	for (ll i = 1; i <= n; i++) {
		cin >> a[i];
	}
	sort(a + 1, a + 1 + n);
	cout<<a[n-k+1];
	return 0;
}