#include <cstdio>

using namespace std;

long long a,b,c,k,i,aux,sol,cnt;
float p;
int main()
{
    FILE *f=fopen("cifre.in","r");
    FILE *g=fopen("cifre.out","w");
    fscanf(f,"%lld %lld",&a,&b);
    fscanf(f,"%lld %lld",&c,&k);
    sol=0;
    for(i=a;i<=b;i++)
    {
        aux=i;
        cnt=0;
        while(aux>0&&cnt<k){if(c==aux%10)cnt++;aux/=10;}
        if(cnt==k)sol++;
    }
    p=(float)sol/(b-a+1);
    fprintf(g,"%.4f",p);
    fclose(f);
    fclose(g);
    return 0;
}