N, D = map(int, input().split())
T, K = [0]*N, [0]*N
for i in range(N):
    T[i], K[i] = map(int, input().split())
dp = [[0]*2 for _ in range(N)]
dp[0][0] = T[0]
dp[0][1] = K[0]-D
for i in range(1,N):
    dp[i][0] = max(dp[i-1][0]+T[i],dp[i-1][1]-D+T[i])
    dp[i][1] = max(dp[i-1][0]-D+K[i],dp[i-1][1]+K[i])
print(max(dp[-1]))