#include<iostream>

using namespace std;

int main(){
  int K,N,F,sum;
  cin >> K >> N >> F;
  sum = K*N;
  int age,needs=0;
  for(int i=0; i<F; i++){
    cin >> age;
    needs += age;
  }
  if(sum < needs){
    cout << -1 << endl;
  }else{
    cout << sum-needs << endl;
  }
}