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

int main() {
  int n, m;
  cin >> n >> m;
  vector<int>a(m);
  int ans = 0;
  rep(i,m) {
    cin >> a[i];
    ans += a[i];
  }
  ans = n-ans;
  if(ans >= 0) cout << ans << endl;
  else cout << -1 << endl;
  
  return 0;
}
