n, D = map(int,input().split())
c = [list(map(int, input().split())) for _ in range(n)]

dp = [[0]*2 for _ in range(n)]
dp[0][0], dp[0][1] = c[0][0], c[0][1]-D
for i in range(1, n):
    dp[i][0] = max(dp[i-1][0]+c[i][0], dp[i-1][1]+c[i][0]-D)
    dp[i][1] = max(dp[i-1][0]+c[i][1]-D, dp[i-1][1]+c[i][1])
print(max(dp[-1]))