help me " [min(ai,an−i+1)+1;max(ai,an−i+1)+k][min(ai,an−i+1)+1;max(ai,an−i+1)+k]. Let’s make +1+1 on this segment using prefix sums (make +1+1 in the left border, −1−1 in the right border plus one and then just compute prefix sums on this array). Let this array be prefpref. Then the value prefxprefx tells the number of such pairs that we need to replace at most one element in this pair to make it sum equals xx."

I dont understand this part of editorial

this is being tricky

plz help @vijju123 @tmwilliamlin admin

thanks in advance