#include<fstream.h>
ifstream f("cifre.in");
ofstream g("cifre.out");
long a,b,c,k,i,nr1,nr,nr2,c1,x,aux;
float r;
int main()
{g.precision(4);
f>>a;
f>>b;
f>>c;
f>>k;
nr2=0;
if(a>b)
{aux=a;
a=b;
b=aux;}
nr1=b-a+1;
for(i=a;i<=b;i++)
{nr=0;
x=i;
while(x>0)
{c1=x%10;
x=x/10;
if(c1==c)
nr++;}
if(nr>=k)
nr2++;}
r=nr2/nr1;
g<<r;
f.close();
g.close();
return 0;
}