sum = 0, i = 2
1: sum = 2
sum = 2, i = 4
2: sum = 6
sum = 6, i = 6
3: sum = 12
sum = 12, i = 8
4: sum = 20
sum = 20, i = 10
5: i => 9, STOP
returns 20

sum = 0, k = 50
1: sum = 50
sum = 50, k = 5
2: sum = 55
sum = 55, k = 0
3: k < 0, STOP
returns 55

sum = 10, p = 15
1: p > 12, STOP
returns 10 

j = 1, increment 1, stop at 3
k = 1, increment 1, stop at less than 3
1: 1 1
2: 1 2
3: 2 1
4: 2 2
5: 3 1
6: 3 2

j = 1, increment 2, stop at 3.
k = 5, decrement 1, stop at more than 3.
1: 1 5
2: 1 4
3: 3 5
4: 3 4