n, d = list(map(int, input().split()))

t, k = list(map(int, input().split()))
tm, km = t, k-d

for _ in range(n-1):
    t, k = list(map(int, input().split()))
    tm, km = t + max(tm, km-d), k + max(tm-d, km)

print(tm if tm >= km else km)