#include <stdio.h>
int A,B,K,C;
int main(){
    freopen("cifre.in","r",stdin);
    freopen("cifre.out","w",stdout);
    scanf("%d %d %d %d",&A,&B,&C,&K);
    int i,c,aux,nr,r=0;
    for (i=A;i<=B;++i){
        aux=i;
        nr=0;
        if (aux==0 && C==0) ++nr;
        while (aux>0){
              c=aux%10;
              aux/=10;
              if (c==C) ++nr;
              }
        if (nr>=K) ++r;
        }
    //printf("%d\n",r);
    printf("%lf",(double)r/(double)(B-A+1));
    return 0;
}