Getting the error as "Only assignment, call, increment, decrement ....." [Resolved]

I am using the below code:-

for (int i = 0; i < str.Length; i + 2)
{
sb.Append(str.Substring(i, 2));
}

I am getting the below error:-
"Only assignment, call, increment, decrement, and new object expressions can be used as a statement"

Can any one help me.

[Resolved]