#include <bits/stdc++.h>
using namespace std;
 
int main(){
  int K,N,F;
  cin>>K>>N>>F;
  vector<int> age(F);
  int sum=0;
  for(int i=0;i<F;i++){
      cin>>age[i];
      sum+=age[i];
  }
  int mame=K*N;
  if(mame<sum){
      cout<<-1<<endl;
  }
  else{
      cout<<mame-sum<<endl;
  }
}