def main():
    N, D = map(int, input().split())
    T = tuple(tuple(map(int, input().split())) for _ in range(N))

    dp = [[0 for i in (0, 1)] for j in range(N)]
    dp[0][0] = T[0][0]
    dp[0][1] = T[0][1] - D
    for i in range(1, N):
        t = T[i][0]
        k = T[i][1]
        dp[i][0] = t + max(dp[i-1][0], dp[i-1][1] - D)
        dp[i][1] = k + max(dp[i-1][1], dp[i-1][0] - D)

    ans = max(dp[N-1][0], dp[N-1][1])
    print(ans)

main()