So as you can see the first bit follows the pattern:
2 x Riff1 + 2 x Riff2 + Riff3
The two parts with singing follow the pattern:
Riff1 + Riff2 + 2xRiff1 + 2xRiff2 + Riff3 (there is a little
embellishment here and there)
But on the very last time lose the last five notes of Riff3 and
sustain the last '1' on the B-string. One other point is when you
play the '2's in Riff3 sustain the note throughout the sequence.