CF29A Spit Problem

In a Berland's zoo there is an enclosure with camels. It is known that camels like to spit. Bob watched these interesting animals for the whole day and registered in his notepad where each animal spitted. Now he wants to know if in the zoo there are two camels, which spitted at each other. Help him to solve this task. The trajectory of a camel's spit is an arc, i.e. if the camel in position $ x $ spits $ d $ meters right, he can hit only the camel in position $ x+d $ , if such a camel exists.

The first line contains integer $ n $ ( $ 1

If there are two camels, which spitted at each other, output YES. Otherwise, output NO.