n,cost = map(int,input().split())
t,k = map(int,input().split())

dp = [[0 for i in range(2)] for j in range(n)]

dp[0][0] = t
dp[0][1] = k - cost

for i in range(1,n):
    
    t,k = map(int,input().split())
    dp[i][0] = t + max(dp[i-1][0],dp[i-1][1] - cost)
    dp[i][1] = k + max(dp[i-1][1],dp[i-1][0] - cost)

print(max(dp[-1][0],dp[-1][1]))