I would like to bring this in notice that the Question: Sum Pairs. https://www.codechef.com/problems/PAIRSUM is not giving AC.(Even if you type old successful codes)
Please do check.
Did you try only submitting only 1 such solution or multiple? Can you please try multiple (3-5)? If they all are giving WA, then we will look into it 
Point is, its possible that test cases got updated to prevent weak solutions, which passed earlier, to pass now.