import sys
readline = sys.stdin.readline

# dp[i][j] = i日目に、j(0:東京、1:京都)で仕事したときの最大値

N,D = map(int,readline().split())
dp = [[0] * 2 for i in range(N + 1)]
dp[0][1] = -D

for i in range(N):
  T,K = map(int,readline().split())
  dp[i + 1][0] = max(dp[i][0], dp[i][1] - D) + T
  dp[i + 1][1] = max(dp[i][1], dp[i][0] - D) + K
  
print(max(dp[N]))