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

int main() {
    int N, M; cin >> N >> M;
    vector<long long> A(M);
    for(int i=0; i<M; ++i) cin >> A[i];
    
    long long sum = accumulate(begin(A), end(A), 0LL);
    if(sum > N) cout << -1 << endl;
    else cout << N - sum << endl;
}