#include <bits/stdc++.h>
#define rep(i, n) for (int i = 0; i < (n); i++)
using namespace std;
typedef long long ll;

int main() {
  int n, m;
  cin >> n >> m;
  vector<int> a(m);
  rep(i, m) cin >> a[i];

  int ans = n;
  rep(i, m) ans -= a[i];

  if (ans < 0) {
    cout << -1 << endl;
  } else {
    cout << ans << endl;
  }
  return 0;
}