n, d = map(int, input().split())
TK = [list(map(int, input().split())) for _ in range(n)]

DP = [[0, 0] for _ in range(n)]
DP[0][0] = TK[0][0]
DP[0][1] = TK[0][1] - d
for i in range(1, n):
    for j in range(2):
        for k in range(2):
            if j == k:
                DP[i][k] = max(DP[i][k], DP[i - 1][j] + TK[i][k])
            else:
                DP[i][k] = max(DP[i][k], DP[i - 1][j] + TK[i][k] - d)
print(max(DP[n - 1]))