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