#include <stdio.h>

int main ( void ) {

	int K , N , F , Data , Sum = 0;
	scanf ( "%d %d %d" , &K , &N , &F );


	for ( int i = 0; i < F; i++ ) {
		scanf ( "%d" , &Data );
		Sum += Data;
	}

	int NumberOfBeans = K * N;
	
	if ( NumberOfBeans < Sum ) printf ( "-1\n");
	else printf ( "%d\n" , NumberOfBeans - Sum );

	return 0;
}