#include <iostream>
using namespace std;

int main(void){
  long long int k, n, f;
  long long int x, sum = 0;
  cin >> k >> n >> f;
  for(int i=0;i<f;i++){
    cin >> x; sum += x;
  }
  if(((k*n) - sum )< 0) cout << "-1" << endl;
  else   cout <<  (k*n) - sum << endl;
}