#include <stdio.h>
#include <stdlib.h>

int main(void)
{
	int k,n;
	short f,*a,i;
	scanf("%d %d %hd",&k,&n,&f);
	a = (short *)malloc(sizeof(short) * f);
	for (i=0;i<f;i++) scanf("%hd",&a[i]);

	k *= n;
	for (i=0;i<f;i++) {
		k -= a[i];
	}
	if (k>=0) printf("%d\n",k);
	else     printf("-1\n");

	return 0;
}