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