Tunnel Entrance and Exit

At the entrance and exit of Namsan Tunnel No. 1, the number of cars passing each minute was recorded. Write a program that finds the maximum number of cars that were inside the tunnel at any moment during the survey.

The first line contains the survey duration in minutes, $n$. The second line contains $m$, the number of cars inside the tunnel at the moment the survey begins. Among the next $n$ lines, the $i$-th line ($1 \le i \le n$) contains the number of cars that passed through the entrance and the number that passed through the exit during the one minute from $(i-1)$ to $i$ minutes after the survey started, in that order. $n \le 10000$, and the number of cars passing in one minute is at most 100.

Let $S_j$ be the number of cars inside the tunnel $j$ minutes after the survey began ($0 \le j \le n$). Print the maximum of $S_0, S_1, \dots, S_n$. However, if the number of cars inside the tunnel ever becomes less than 0, print 0 instead.