#include <iostream>

using namespace std;
int main(){
	int K,N,F,A = 0;
  cin >> K >> N >> F;
  for(int i=0;i<F;++i){
    int a;
    cin >> a;
    A += a;
  }
  int ans = K*N - A;
  if(ans < 0){
    std::cout << -1 << std::endl;
  }else{
    std::cout << ans << std::endl;
  }
  return 0;
}