import sys
import pypyjit
import itertools
import heapq
import math
from collections import deque, defaultdict
import bisect

input = sys.stdin.readline
sys.setrecursionlimit(10 ** 6)
pypyjit.set_param('max_unroll_recursion=-1')


def index_lt(a, x):
    'return largest index s.t. A[i] < x or -1 if it does not exist'
    return bisect.bisect_left(a, x) - 1


def index_le(a, x):
    'return largest index s.t. A[i] <= x or -1 if it does not exist'
    return bisect.bisect_right(a, x) - 1


def index_gt(a, x):
    'return smallest index s.t. A[i] > x or len(a) if it does not exist'
    return bisect.bisect_right(a, x)


def index_ge(a, x):
    'return smallest index s.t. A[i] >= x or len(a) if it does not exist'
    return bisect.bisect_left(a, x)


N, D = map(int, input().split())
reward = [tuple(map(int, input().split())) for _ in range(N)]

INF = 1 << 30
dp = [[0, 0] for _ in range(N + 1)]
t1, k1 = reward[0]
dp[1] = [t1, k1 - D]

for i in range(1, N):
    t, k = reward[i]
    dp[i + 1][0] = max(dp[i][0] + t, dp[i][1] + t - D)
    dp[i + 1][1] = max(dp[i][0] + k - D, dp[i][1] + k)
print(max(dp[N]))