AT_abc424_c [ABC424C] New Skill Acquired

Takahashi is playing a game. This game has $ N $ skills numbered $ 1 $ through $ N $ . You are given $ N $ pairs of integers $ (A_1,B_1), \dots,(A_N,B_N) $ . If $ (A_i,B_i)=(0,0) $ , then Takahashi has already learned skill $ i $ . Otherwise, Takahashi can learn skill $ i $ if and only if at least one of skills $ A_i $ and $ B_i $ has already been learned. Including the skills already learned, find the number of skills that Takahashi can ultimately learn.

The input is given from Standard Input in the following format: > $ N $ $ A_1 $ $ B_1 $ $ A_2 $ $ B_2 $ $ \vdots $ $ A_N $ $ B_N $

Output the answer.

### Sample Explanation 1 At first, Takahashi has already learned skill $ 1 $ . Because skill $ 1 $ has been learned, skill $ 2 $ can be learned, and learning skill $ 2 $ enables learning skill $ 3 $ . Since it is impossible to learn skills $ 4,5,6 $ , the answer is $ 3 $ . ### Constraints - $ 1\leq N \leq 2\times 10^5 $ - $ (A_i,B_i)=(0,0) $ or $ 1\leq A_i,B_i \leq N $ - All input values are integers.