#include <bits/stdc++.h>

using namespace std;

int main(){
  int k,n,f;
  cin >> k >> n >> f;
  int humans[f];
  int totalBeans = k*n;
  int needBeans = 0;

  for(int i = 0; i < f; i++){
    cin >> humans[i];
    needBeans += humans[i];
  }
  
  int ans = totalBeans - needBeans;
  if(ans < 0){
    cout << -1 << endl;
  }else{
    cout << ans << endl;
  }

  return 0;
}