#include <bits/stdc++.h>
using namespace std;
typedef long long ll;

int main(){
	int K, N, F;
	cin >> K >> N >> F;

	int rem = K * N;
	while(F--){
		int A; cin >> A;
		rem -= A;
	}

	cout << max(rem, -1) << endl;
	return 0;
}