import sys
input = sys.stdin.readline

N,D=map(int,input().split())

DP=[0,-10**100]

for i in range(N):
    NDP=[0,0]
    t,k=map(int,input().split())

    NDP[0]=max(DP[0]+t,DP[1]+t-D)
    NDP[1]=max(DP[1]+k,DP[0]+k-D)

    DP=NDP

print(max(DP))