    #include <bits/stdc++.h>
    using namespace std;
     
    int main() {
      		unsigned long long N;
      int M;
      cin >> N >> M;
      vector<unsigned long long> A[M];
      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";
       }
    }