#include<stdio.h>
int main()
{int a,b,c,k,l=0, aux;
float p, id=0.0000, pos;
FILE *f=fopen("cifre.in", "r");
FILE *g=fopen("cifre.out", "w");
fscanf(f,"%d", &a);
fscanf(f, "%d", &b);
fscanf(f, "%d", &c);
fscanf(f, "%d", &k);
pos=b-a+1;
for(a; a<=b; a++)
	{aux=a;
	l=0;
	while(aux!=0)
		{if(aux%10==c) {l++;}
		aux=aux/10;}
	if(l>=k) id++;
	}

p=id/pos;

fprintf(g, "%f", p);
fclose(f);
fclose(g);
return 0;
}