#!/usr/bin/env python3


東京 = 0
京都 = 1


def solve(n, d, wage):
    dp = [[0, 0] for _ in range(n + 1)]
    dp[0][東京] = 0
    dp[0][京都] = -d  # 1日目の前に入洛したとみなす
    for i in range(n):
        for p in (東京, 京都):
            dp[i + 1][p] = max(dp[i + 1][p], dp[i][p] + wage[i][p])
            dp[i + 1][1 - p] = max(dp[i + 1][1 - p],
                                   dp[i][p] + wage[i][1 - p] - d)
    return max(dp[n])


def main():
    n, d = map(int, input().split())
    wage = [list(map(int, input().split())) for _ in range(n)]
    res = solve(n, d, wage)
    print(res)


if __name__ == '__main__':
    main()