question : C - Sum of product of pairs
editorial : Editorial - AtCoder Beginner Contest 177
if (sum < 0) sum += mod;
what does this line of code do?
for negative sum you have to add mod first to ensure it doesn’t remain negative and then perform mod on it , Because mod works differently for negative and positive
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Can you help me with this problem of the same contest.
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what if i use abs() ? 
I will give you an example? As the definition of mod says it is the smallest number you should subtract from that number to make it divisible by number you are performing mod with , so ans for a simple 2 mod 3 will result out as 2 which means if we subtract 2 from 2 you make it divisible by 3 , now what if I perform -2 mod 3 this will result out as -2 , again see if you subtract -2 from -2 you will get it divisible by 3 But Actually our result has parted ways with convention we follow in question which require the ans in terms of modulo it should stay between 0 and MOD-1 , We should go for the value 1 instead if we subtract 1 from -2 we get -3 which is divisible by 3 so that is why we add mod for negative cases . ( -2 + 3) mod 3 = > 1 mod 3 => 1 (The answer we need )
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Sorry , I have not upsolved it bro
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ohh…got it bro! and thank you so much 
ohk
its fine bro 