import sequtils,strutils,math
var
    N, D : int
    T, K : int
    dp : array[110, array[2, int]]
    
(N, D) = stdin.readline.split.map(parseInt)
dp[0][1] = -D
for n in 1..N:
    (T, K) = stdin.readline.split.map(parseInt)
    dp[n][0] = max(dp[n - 1][0] + T, dp[n - 1][1] + T - D)
    dp[n][1] = max(dp[n - 1][1] + K, dp[n - 1][0] + K - D)
echo max(dp[N])