#include <bits/stdc++.h>
#define int long long 
using namespace std;
using Graph = vector<vector<int>>;

signed main(){
  int K, N, F;
  cin >> K >> N >> F;
  
  int B = K*N;
  for( int i = 0; i < F; i++ ){
    int A;
    cin >> A;
    B -= A;
  }
  if( B < 0 ) cout << -1 << endl;
  else cout << B << endl;
}