#include<fstream.h>
ifstream f("cifre.in");
ofstream g("cifre.out");
long a,b,nr,c1,c,nr1,k,i,nrr,aux,da;
float r;
int main()
{f>>a;
f>>b;
f>>c;
f>>k;
nr=0;
if(a>=b)
{aux=a;
a=b;
b=aux;}
nr1=b-a+1;
for(i=a;i<=b;i++)
{nrr=0;
while(i>0)
{c1=i%10;
i=i/10;
if(c1==c)
nrr++;}
if(nrr>=k)
nr++;}
r=nr1/nr;
da=r*10000;
r=da/10000;
g<<da<<endl;
f.close();
g.close();
return 0;}