 
#include <bits/stdc++.h>
using namespace std;
 
int main() {
  		unsigned long long N;
  int M;
  cin >> N >> M;
  vector<unsigned long long> A;
  unsigned long long sum = 0;
  for (int i= 0;i<M;++i){
    cin >> A[i];
    sum += A[i];
  }
  if(N - sum > 0){
    cout << N - sum;
  }else{ 
    cout << "-1";
   }
}