#include<stdio.h>
#include<stdlib.h>
#include<math.h>
unsigned long a,b,i,d; unsigned int c,k,q,Q=1; double p;
int main()
{
    FILE*f=fopen("cifre.in","r");
    FILE*g=fopen("cifre.out","w");
    fscanf(f,"%lu %lu %u %u",&a,&b,&c,&k);
    for(i=a;i<=b;i++)
    {
        d=i; q=0;
        while(d!=0&&q<=k)
        {
            if(d%10==k)q++;
            d/=10;
        }
        if(k==q) Q++;
    }
    p=(float)Q/b;
    fprintf(g,"%.*lf",4,p);
    fclose(f);
    fclose(g);
    return 0;
}