# No.561 東京と京都
import sys
input = sys.stdin.readline

def main():
    N, D = tuple(map(int, input().rstrip().split()))
    A = tuple(tuple(map(int, input().rstrip().split())) for _ in range(N))
    dp = [[0] * N for _ in range(2)]
    dp[0][0], dp[1][0] = A[0][0], A[0][1] - D
    for i in range(1, N):
        t, k = A[i]
        dp[0][i] = max(dp[0][i - 1] + t, dp[1][i - 1] + t - D)
        dp[1][i] = max(dp[0][i - 1] + k - D, dp[1][i - 1] + k)
    print(max(dp[0][-1], dp[1][-1]))


if __name__ == "__main__":
    main()