#include<stdio.h>	
int main()
{
	int k, n, f;
	int str[20];
	int a;
	int count=0;
	int remain;
	scanf("%d%d%d", &k, &n, &f);
	for (int i = 0; i <= f - 1; i++) {
		scanf("%d", &(str[i]));
		count += str[i];
	}
	remain = k*n - count;
	if (remain >= 0)
		printf("%d", remain);
	else
		printf("%d", -1);
	return 0;
}