#include <iostream>
using namespace std;

int main(){
  int k,n,f;
  cin >>k>>n>>f;
  int num=k*n,age;
  for(int i=0;i<f;i++){cin >> age;num-=age;}
  if(num<0)cout << -1 << endl;
  else cout << num << endl;

  return 0;
}