#include "bits/stdc++.h"
using namespace std;


int main() {
	int K,N,F;
	cin >> K >> N >> F;
	vector<int> A(F);
	int sumA=0;
	for (int i = 0; i < F; i++) {
		cin >> A[i];
		sumA += A[i];
	}

	int sum = N*K;

	if(sum-sumA<0) cout << -1 << endl;
	else cout << sum-sumA << endl;

}