program cifre1;
var a,b,c,k,i,j,aux,nrpos,nrsol,aux1,aux2:longint;
    sol:real;
    ok:boolean;
begin
assign(input,'cifre.in');
reset(input);
readln(input,a,b,c,k);
close(input);
assign(output,'cifre.out');
rewrite(output);
nrpos:=b-a+1;
for i:=a to b do
begin
aux:=i;
aux1:=0;
while(aux<>0)and(aux1<=k) do
begin
j:=aux mod 10;
if j=c then inc(aux1);
aux:=aux div 10;
end;
if (aux1>=k) then inc(nrsol);
end;
sol:=nrsol/nrpos;
write(output,sol:10:4);
end.