[contest:https://mirror.codeforces.com/contest/2178]

Key Observation:

------------------ 1.The "Yes" is Persistent: In the rules, Y OR N = Y. This means a Y never disappears; it just "**eats**" the N next to it. 2.The Trap: If you have two Ys, they will eventually have to meet as the string gets shorter. Because a Y cannot be turned back into an N, those two Ys will eventually be forced to merge. 3.The Rule: If the string has 0 or 1 Y, you can always merge all N's together first, then merge that single N with the single Y. But if there are greater than equal (>=) 2 Ys, you will eventually be forced to break the rule.