#include <bits/stdc++.h>
using namespace std;
int main (){
	
	cin.tie(0);
	ios::sync_with_stdio(false);
	
	int K,N,F;
	cin >> K >> N >> F;
	int total = K*N;
	vector<int> vc(F);
	int res{0};
	for( auto& x: vc ){
		cin >> x;
		res+=x;
	}
	cout << ( total < res ? -1 : total - res ) << endl;
	return 0;
}