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

int main()
{
int i;
int K;
int N;
int F;
int A;
int iB;

	cin >> K;
	cin >> N;
	cin >> F;

	iB = K * N;
	for( i = 0; i < F; i++ )
	{
		cin >> A;
		iB -= A;
	}

	if( iB >= 0 )
		cout << iB << endl;
	else
		cout << - 1 << endl;

	return 0;
}