#include <stdio.h>
#include <math.h>

int main()
{
    int a,b,k,c,d,n,rez;
    float prb;
    FILE *in=fopen("cifre.in","r");
    FILE *out=fopen("cifre.out","w");
    fscanf(in,"%d%d%d%d",&a,&b,&k,&c);
    d=b-a+1;
    n=0;
    rez=0;
    while(a<=b){
        int kkt=a;
        while(kkt>0){
            if(kkt%10==c) n++;
            kkt=kkt/10;
        }
        if(n>=k) rez++;
        n=0;
        a++;
    }
    prb=(float)rez/(float)d;
    prb=floor(10000*prb)/10000;
    fprintf(out,"%f",prb);
    return 0;
}