# -*- coding: utf-8 -*-

N,D = map(int, input().split())
T = [-1]
K = [-1]
for i in range(N):
    t,k = map(int, input().split())
    T.append(t)
    K.append(k)

dp = [[-1,-1]]
for i in range(N):
    dp.append([-10000001,-10000001])

# nは何日目か，pはn日目の仕事前にどこにいるか（0:東京, 1:京都）
def solution(n, p):
    if dp[n][p] > -10000001:
        return dp[n][p]

    price = -1
    if n == N:
        if p==0:
            price = max(T[n], K[n]-D)
        else:
            price = max(T[n]-D, K[n])
    else:
        if p==0:
            price = max(T[n]+solution(n+1,0), K[n]-D+solution(n+1,1))
        else:
            price = max(T[n]-D+solution(n+1, 0), K[n]+solution(n+1, 1))
    
    dp[n][p] = price
    return dp[n][p]

print(solution(1, 0))