#include<stdio.h>
int n,i,k,m,a,b,s,l;
void read()
{scanf("%d%d%d%d",&n,&m,&a,&b);
}
int cifre(int x)
{
    int cf=0;
    while(x)
   { if(x%10==b)
    cf++;
    x/=10;
}
    return cf;

}   
void solve()
{
     for(i=n;i<=m;i++)
     if(cifre(i)>=a)
     s++;
}
int main ()
{
    freopen("cifre.in","r",stdin);
    freopen("cifre.out","w",stdout);
    read();
    solve();
    float p;
    p=(float)s/(m-n+1);
    printf("%.4f\n",p);
    return 0;
}