import numpy as np
N, D = map(int, input().split())
T, K = map(int, input().split())

dp = np.zeros((N, 2), dtype=int)
dp[0][0] = T
dp[0][1] = K

for i in range(1, N):
    T, K = map(int, input().split())
    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)
    
print(max(dp[N-1]))