hackathon: pb: "programming contest"(3)

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

signed main() {
    //freopen("text.in", "r", stdin);
    int n, sum = 0;
    cin >> n;
    for (int x; cin >> x;) {
        cerr << x << endl;
        sum += x;
    }
    int ans = n * (n + 1) / 2 - sum;
    if (!ans) {
        puts("NU");
    } else {
        cout << ans << endl;
    }
    return 0;
}