Wire Net Equality Constraints Feasible

A circuit checker has e constraints over wire nets, each net named by a single lowercase letter (a-z). Each constraint is exactly four characters: a net letter, then either == or !=, then another net letter. x==y means nets x and y must carry the same signal; x!=y means they must carry different signals.

Decide whether there is any assignment of signals to nets that satisfies every constraint. Print YES if one exists, otherwise NO.

Input format

• Line 1: an integer e, the number of constraints.
• Next e lines: one constraint each, of the form x==y or x!=y (four characters).

Output format

YES if all constraints can be satisfied simultaneously, otherwise NO.

Constraints

• 1 <= e <= 100000
• Every net name is a single lowercase English letter.