#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() {
  ll n, m;
  cin >> n >> m;
  vector<ll> a(m);
  rep(i, m) cin >> a[i];
  rep(i, m) {
    n -= a[i];
    if(n < 0 ){
      cout << "-1" << endl;
      return 0;
    }
  }
  cout << n << endl;
  return 0;
}