Stupid sequence #2

It's an alternate pattern. The first pattern goes like this

$2,3,5,7,11,?$ This is clear that it's a series of prime numbers. The next number is 13.

The second pattern goes like

$3,5,4,5,6,?$ This is the one to crack. This is the number of letters in the preceding prime number. thus the next number is the number of letters in THIRTEEN=8

$\sum _{ n=1 }^{ 1 }{ 8 } +\quad \sum _{ n=1 }^{ 1 }{ 13 } =\quad 21$