# -*- coding=utf-8 -*-

if __name__ == "__main__":
    N, D = list(map(int, input().split(" ")))
    DP = []
    for n in range(N):
        DP.append([0, 0])

    for n in range(N):
        T, K = list(map(int, input().split(" ")))
        if n == 0:
            DP[n][0] = T
            DP[n][1] = K - D
        else:
            DP[n][0] = max(T + DP[n - 1][0], (T - D) + DP[n - 1][1])
            DP[n][1] = max(K + DP[n - 1][1], (K - D) + DP[n - 1][0])
    
    print(max(DP[-1]))