#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
#include <ext/pb_ds/hash_policy.hpp>
using namespace std;
using namespace __gnu_pbds;
#ifdef LOCAL
#include "algo/debug.h"
#else
#define debug(...) 42
#endif
typedef long long ll;
typedef pair < int, int > PII;
typedef int itn;
#define rep(i, a, b) for(int i = a;i <= b; ++ i)
#define per(i, b, a) for(int i = b;i >= a; -- i)
mt19937 RND_MAKER (chrono :: steady_clock :: now ().time_since_epoch ().count ());
inline ll randomly (const ll l, const ll r) {return (RND_MAKER () ^ (1ull << 63)) % (r - l + 1) + l;}
#define int long long
namespace fastIO {
	template < typename T > void read (T &x) {x = 0; T flag = 1; char c = getchar (); for ( ; c < '0' || c > '9' ; c = getchar ()) flag = (c == '-') ? -1 : flag; for ( ; c >= '0' && c <= '9' ; c = getchar ()) x = (x << 1) + (x << 3) + (c ^ 48); x *= flag;}
	template < typename T, typename ...T1 > void read (T &x, T1 &...x1) {read (x); read (x1...);}
	template < typename T > void write (T x) {if (x < 0) {x = -x; putchar ('-');} if (x / 10) {write (x / 10);} putchar (x % 10 + '0');}
	template < typename T > void writesp (T x) {write (x); putchar (' ');}
	template < typename T > void writeln (T x) {write (x); putchar ('\n');}
}
using namespace fastIO;
const double pi = acos (-1);
//__gnu_pbds :: tree < Key, Mapped, Cmp_Fn = std :: less < Key >, Tag = rb_tree_tag, Node_Upadte = null_tree_node_update, Allocator = std :: allocator < char > > ;
//__gnu_pbds :: tree < PPS, __gnu_pbds :: null_type, less < PPS >, __gnu_pbds :: rb_tree_tag, __gnu_pbds :: tree_order_statistics_node_update > tr;
const int N = 100005;
int n, W, a[N], b[N], sa[N], sb[N];
signed main () {
	read (n, W);
	rep (i, 1, n) read (a[i]);
	rep (i, 1, n) read (b[i]);
	sort (a + 1, a + 1 + n);
	reverse (a + 1, a + 1 + n);
	sort (b + 1, b + 1 + n);
	reverse (b + 1, b + 1 + n);	
	rep (i, 1, n) {
		sa[i] = sa[i - 1] + a[i];
		sb[i] = sb[i - 1] + b[i];
	}
	int ans = -1e18;
	rep (i, 1, n) {
		int mn = sa[i], mx, c = 0;
		if (sb[n] < mn) continue;
		int L = 1, R = n;
		while (R - L > 1) {
			int mid = L + R >> 1;
			if (sb[mid] >= mn) R = mid;
			else L = mid;
		}
		if (sb[L] >= mn) mx = sb[L], c += W * L;
		else mx = sb[R], c += W * R;
		c += W * i;
		ans = max (ans, mn - c);
	}
	rep (i, 1, n) {
		int mn = sb[i], mx, c = 0;
		if (sa[n] < mn) continue;
		int L = 1, R = n;
		while (R - L > 1) {
			int mid = L + R >> 1;
			if (sa[mid] >= mn) R = mid;
			else L = mid;
		}
		if (sa[L] >= mn) mx = sa[L], c += W * L;
		else mx = sa[R], c += W * R;
		c += W * i;
		ans = max (ans, mn - c);
	}	
	writeln (ans);
	return 0;
}