#include<stdio.h>
int main()
	{
	FILE*f,*g;
	int a,b,c,k;
	f=fopen("cifre.in","r");
	g=fopen("cifre.out","w");
	fscanf(f,"%d %d %d %d",&a,&b,&c,&k);
	int q=0,d;
	for(int i=a;i<=b;i++)
		{int contor=0;
		int x=i;
		while(x)
			{d=x%10;
			x=x/10;
			if(d==c) contor++;}
		if(contor>=k) q++;}
	float v=(float)q/(float)(b-a+1);
	fprintf(g,"%.4f",v);
	return 0;
	}