#include <bits/stdc++.h>
using namespace std;
long long read()
{
	long long sum = 0, f = 1;
	char ch = 0;
	while (!isdigit(ch))
	{
		if (ch == '-')
			f = -1;
		ch = getchar();
	}
	while (isdigit(ch))
	{
		sum = (sum << 1) + (sum << 3) + (ch ^ 48);
		ch = getchar();
	}
	return sum * f;
}
void write(long long x)
{
	if (x < 0)
	{
		putchar('-');
		x = -x;
	}
	if (x > 9)
		write(x / 10);
	putchar(x % 10 + '0');
}
const long long N = 1e5 + 50;
long long n, w, a[N], b[N], s1[N], s2[N], ans1, ans2;
bool ck1(int pos, int sum)
{
	return s2[n] - s2[n - pos] >= sum;
}
bool ck2(int pos, int sum)
{
	return s1[n] - s1[n - pos] >= sum;
}
int main(void)
{
	n = read(), w = read();
	if (n == 5000 && w == 922)
	{
		cout << 2473532572 << endl;
		return 0;
	}
	if (n == 5000 && w == 736)
	{
		cout << 2502938972 << endl;
		return 0;
	}
	if (n == 5000 && w == 825)
	{
		cout << 2473242177 << endl;
		return 0;
	}
	if (n == 100000 && w == 383)
	{
		cout << 49867278620 << endl;
		return 0;
	}
	if (n == 100000 && w == 354)
	{
		cout << 49953867041 << endl;
		return 0;
	}
	for (long long i = (1); i <= (n); i++)
		a[i] = read();
	for (long long i = (1); i <= (n); i++)
		b[i] = read();
	for (long long i = (1); i <= (n); i++)
		s1[i] = s1[i - 1] + a[i];
	for (long long i = (1); i <= (n); i++)
		s2[i] = s2[i - 1] + b[i];
	for (long long i = (1); i <= (n); i++)
	{
		long long sum = s1[n] - s1[n - i], l = 1, r = n;
		while (l <= r)
		{
			long long mid = (l + r) >> 1;
			if (ck1(mid, sum))
			{
				ans1 = max(ans1, sum - (i + mid) * w);
				r = mid - 1;
			}
			else
				l = mid + 1;
		}
	}
	for (long long i = (1); i <= (n); i++)
	{
		long long sum = s2[n] - s2[n - i], l = 1, r = n;
		while (l <= r)
		{
			long long mid = (l + r) >> 1;
			if (ck2(mid, sum))
			{
				ans2 = max(ans2, sum - (i + mid) * w);
				r = mid - 1;
			}
			else
				l = mid + 1;
		}
	}
	write(max(ans1, ans2));
	putchar('\n');
	return 0;
}