#include<fstream>
using namespace std;
ifstream f("cifre.in");
ofstream g("cifre.out");
long long x,p,n,a,b,k,c;
long long nr;
int cifre(int n)
{int nr=0;
for(;n;n/=10)
    if(n%10==c)
        ++nr;
return nr;
}
int main()
{f>>a>>b>>c>>k;
if(b-a>=10000)
{for(;a%10000;++a,p++)
{if(cifre(a)>=k)
    ++nr;
}
for(x=0;b-a>=10000;a+=10000,p+=10000)
{x=cifre(a/10000);
if(x>=k)
    nr+=6561;
if(x>=k-1)
    nr+=2916;
if(x>=k-2)
    nr+=487;
if(x>=k-3)
    nr+=35;
if(x>=k-4)
    ++nr;
}
}
for(;a<=b;++a,++p)
    if(cifre(a)>=k)
        ++nr;
g<<(double)nr/p;
return 0;
}