#include <stdio.h>

int main (void){
	int k,n,f,t;
	int a[100];
	scanf("%d%d%d",&k,&n,&f);
	t = k*n;
	for(int i = 0;i < f;i++) {
		scanf("%d",&a[i]);
		if(t >= a[i]) t -= a[i];
		else {
			t = -1;
		}
	}
	printf("%d",t);
	return(0);
}