#include <bits/stdc++.h>
#define rep(i,n) for (int i = 0; i < (n); ++i)
using namespace std;
using ll = long long;
using P = pair<int,int>;
 
int main(){
    int n, m, in;
    cin >> n >> m;
    rep(i, m){
        cin >> in;
        n -= in;
    }
    if(n >= 0){
        cout << n << endl;
    }else{
        cout << -1 << endl;
    }
    return 0;
}