#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;

int main(){
  // input
  int K,N,F;
  cin >> K >> N>> F;
  unsigned long long  A[F+1];


  unsigned long long  totalbeans = 0;
  for(int i=1;i<=F;i++){
    cin >> A[i];
    totalbeans += A[i];
  }
  int ans;
  if(K*N >= totalbeans){
    ans = K*N - totalbeans;
  } else {
    ans = -1;
  }


  cout << ans << endl;
  // output 
}