Judging from their YEAR of birth, add the last two digits:

Leeteuk:83...8+3=11....1+1=2

Heechul:83...8+3=11....1+1=2

Hankyung:84...8+4=12....1+2=3

Yesung:84...8+4=12....1+2=3

Kangin:85...8+5=13....1+3=4

Shindong:85...8+5=13....1+3=4

Sungmin:86...8+6=14....1+4=5

Eunhyuk:86...8+6=14....1+4=5

Donghae:86...8+6=14....1+4=5

Siwon:87...8+7=15....1+5=6

Ryeowook:87...8+7=15....1+5=6

Kibum:87...8+7=15....1+5=6

Khyunyun:88...8+8=16....1+6=7

Then Add All The Results Obtained

2+2+3+3++4+4+5+5+5+6+6+6+7=58

5+8=13