#include <iostream>
#include <vector>

using namespace std;

int main()
{
	int K, N, F;
	cin >> K >> N >> F;
	int sum = 0;

	vector<int> A(F);
	for( int i = 0; i < F; i++ ) {
		cin >> A.at(i);
		sum += A.at(i);
	}
	int a = K * N - sum;
	if( a >= 0 ) {
		cout <<  a << endl;
	}
	else {
		cout << "-1" << endl;
	}
	return 0;
}