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

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

	int sum = 0;
	for( int i = 0; i < F; i++ ) {
		int A;
		cin >> A;
		sum += A;
	}

	int ans = K * N - sum;
	if( ans < 0 ) ans = -1;

	cout << ans << endl;
}