#include <iostream>
#include <fstream>
using namespace std;

int main(){


  int k,n,f,result,sumf;
  cin >> k >> n >> f;
  sumf = 0;
  for (int i = 0; i < f; i++) {
    int a;
    cin >> a;
    sumf += a;
  }
  result = k * n - sumf;
  if (result < 0) {
    cout << -1 << "\n";
  }else{
    cout << result << "\n";
  }
  return 0;
}