n = int(input())
t = int(input())
razn = 0
a1,b1,a,b = 0,0,0,0
for i in range(n):
    a1,b1 = [int(x) for x in input().split()]
    a += a1
    b += b1
    if a - b < razn :
        razn = a - b
print(b + razn + t)