#include <bits/stdc++.h>
using namespace std;
#define int long long
signed main(){
  int K,N,F;
  cin>>K>>N>>F;
  int sum = 0;
  for(int i=0;i<F;i++){
      int x;
      cin>>x;
      sum += x;
  }
  cout<<max(-1ll,K*N-sum)<<endl;
}