#include<iostream.h>
#include<fstream.h>
int cif(int i,int c)
{int nr=0;
while(i)
{
 if(i%10==c)nr++;
 i/=10;
}
return nr;
}

int main()
{unsigned long a,b,i,c,k,r=0,y=0;
ifstream f("cifre.in");
ofstream g("cifre.out");
f>>a;
f>>b;
f>>k;
f>>c;
for(i=a;i<=b;i++)
if(cif(i,c)>=k)r++;
y=b-a+1;
g<<(float)r/y;
f.close();
g.close();
return 0;
}