N, K = map(int, input().split())
pos = 0
ans = 0
AB = []
for _ inrange(N):
A, B = map(int, input().split())
AB.append((A, B))
AB = sorted(AB, key=lambda x: x[0])
for A, B in AB:
if A - pos > K:
ans = K + pos
K = 0breakelse:
K = K + B - (A-pos)
pos = A
if K > 0:
ans = K + pos
print(ans)