#include<stdio.h>  
int main()  
{  
	long A,B,i,x;
	int c,k,nr;
	float p,j=0;
	freopen("cifre.in","r",stdin);
    freopen("cifre.out","w",stdout);	
	scanf("%ld",&A); scanf("%ld",&B); scanf("%d",&c); scanf("%d",&k);
	for (i=A;i<=B;i++)
	{
		nr=0;
		x=i;
		while (x!=0)
		{
			if (x%10==c) nr+=1;
			x/=10;
		}
		if (k<=nr) j+=1;
	}
	p=j/(B-A+1);
	printf("%f",p);
    return 0;
}