Change the varaible in For Loop

[quest]

Hello,
I have a very simple problem:
I am trying to increase the i in for loop like that:
1 2
4 5
3 4
6 7
5 6
8 9
One step I am increasing the i with +3 and the other step I am decreasing the i with -1

for i in range(1,number_of_qubit-2):
       print(i,i+1)
       i=i+3
       print(i,i+1)
       i=i-1

However I am getting this result:
1 2
4 5
2 3
5 6
3 4
6 7
4 5
7 8
Why am I seeing 2 and 3 instead of 4 and 5 ? How can I fix this problem