#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)

int main() {
  int k,n,f;
  cin>>k>>n>>f;
  int sum=k*n;
  rep(i,f){
  	int a;
  	cin>>a;
  	sum-=a;
  	if(sum<0){
  		cout<<-1<<endl;
      exit(0);
    }
  }
  cout<<sum<<endl;
}