#include<bits/stdc++.h>
using namespace std;
#define int long long

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