#Python 3.5.2
s = 0.0; b = int(input("b = "))
for x in range(3, 19+1): s+=(4*x)/(b*x);
print ("S =", s)
//FPC 2.6.4
var
i1, i2, x: integer;
b, s: real;
begin
s:=0;
write ('B = '); readln (b);
i1:=3; i2:=19;
for x:=i1 to i2 do s:=s+(4*x)/(b*x);
write ('S = ', s);
end.