18

The piano melody of this song goes: 
C G C D E 
Bm F# B C# D 
G E G A B 
Em B E F# G 
D A D E F# 
C G C D   E 
 
Every chord is followed by its 5th in the lower octave, 
then the key itself, the 2nd and 3rd (or 3rd minor), 
which are all shown above