n, d = map(int, input().split())
TK = []
for i in range(n):
    t, k = map(int, input().split())
    TK.append((t, k))

dp = [0, 0]
for i, (t, k) in enumerate(TK):
    nx = [0, 0]
    if i == 0:
        nx[0] = t
        nx[1] = k-d
    else:
        nx[0] = max(dp[0]+t, dp[1]+t-d)
        nx[1] = max(dp[0]+k-d, dp[1]+k)
    dp = nx
print(max(dp))