#region Header
#!/usr/bin/env python3
# from typing import *

import sys
import io
import math
import collections
import decimal
import itertools
from queue import PriorityQueue
import bisect
import heapq

def input():
    return sys.stdin.readline()[:-1]

sys.setrecursionlimit(1000000)
#endregion

# _INPUT = """2 10000
# 20000 32000
# 24000 23000
# """
# sys.stdin = io.StringIO(_INPUT)


def main():
    N, D = map(int, input().split())
    T = [None for _ in range(N)]
    K = [None for _ in range(N)]
    for i in range(N):
        T[i], K[i] = map(int, input().split())

    dp = [[0, 0] for _ in range(N)]
    dp[0][0] = T[0]
    dp[0][1] = K[0] - D
    for i in range(1, N):
        dp[i][0] = max(dp[i-1][0] + T[i], dp[i-1][1] + T[i] - D)
        dp[i][1] = max(dp[i-1][0] + K[i] - D, dp[i-1][1] + K[i])
    print(max(dp[N-1]))


if __name__ == '__main__':
    main()