#include <stdio.h>
int main(void){
    int k, n, f;
	scanf("%d %d %d", &k, &n, &f );
    //int age[f];
    int age;
    int taberu = 0;
	for( int i = 0; i < f; i++ ) {
		scanf( "%d", &age );
		taberu += age;
	}
    printf( "%d\n", ( k * n - taberu ) >= 0 ? k * n - taberu : -1 );
}