CF608A Saitama Destroys Hotel

Saitama accidentally destroyed a hotel again. To repay the hotel company, Genos has volunteered to operate an elevator in one of its other hotels. The elevator is special — it starts on the top floor, can only move down, and has infinite capacity. Floors are numbered from $ 0 $ to $ s $ and elevator initially starts on floor $ s $ at time $ 0 $ . The elevator takes exactly $ 1 $ second to move down exactly $ 1 $ floor and negligible time to pick up passengers. Genos is given a list detailing when and on which floor passengers arrive. Please determine how long in seconds it will take Genos to bring all passengers to floor $ 0 $ .

The first line of input contains two integers $ n $ and $ s $ ( $ 1

Print a single integer — the minimum amount of time in seconds needed to bring all the passengers to floor $ 0 $ .

In the first sample, it takes at least $ 11 $ seconds to bring all passengers to floor $ 0 $ . Here is how this could be done: 1\. Move to floor $ 5 $ : takes $ 2 $ seconds. 2\. Pick up passenger $ 3 $ . 3\. Move to floor $ 3 $ : takes $ 2 $ seconds. 4\. Wait for passenger $ 2 $ to arrive: takes $ 4 $ seconds. 5\. Pick up passenger $ 2 $ . 6\. Go to floor $ 2 $ : takes $ 1 $ second. 7\. Pick up passenger $ 1 $ . 8\. Go to floor $ 0 $ : takes $ 2 $ seconds. This gives a total of $ 2+2+4+1+2=11 $ seconds.