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

int main(){
  int n, m, a, sum=0;
  cin >> n >> m;
  for(int i=0;i<m;i++){
    cin >> a ;
    sum = sum + a;
  }
  if(sum<=n)
    cout << n-sum << endl;
  else
    cout << -1 << endl;
}