#include<stdio.h>
int main()
{
	int K,N,F;
	int A[100],Msum,Ksum=0;	//“¤sum,‰Ć‘°sum
	int i;
	
	scanf("%d%d%d",&K,&N,&F);
	for(i=0;i<F;i++)
		scanf("%d",&A[i]);
	
	Msum=K*N;
	for(i=0;i<F;i++)
		Ksum+=A[i];
	
	if(Msum>=Ksum)
		printf("%d",Msum-Ksum);
	else
		printf("-1");
	
	return 0;
}