N,D = map(int,input().split())
dp = [0] * 2
dp[1] = -(1 << 60)
for i in range(N):
    t,k = map(int,input().split())
    new_dp = dp.copy()
    new_dp[0] = max(dp[0] + t, dp[1] + t - D)
    new_dp[1] = max(dp[1] + k, dp[0] + k - D)
    dp = new_dp
    
print(max(dp))