# include <stdio.h>
int main(void){
	int k,n,f;
	scanf("%d %d %d",&k,&n,&f);
	int a,b,r;
	b=k*n;
	for(int i=0;i<f;i++){
		scanf("%d",&r);
		a+=r;
	}
	r=-1;
	if(b-a>=0)r=b-a;
	printf("%d\n",r);
}