# include <bits/stdc++.h>
# define pb push_back
# define ff first
# define ss second
# define nl "\n"
# define sz(x) ((int)(x).size())
# define deb(x) cerr << #x << " = " << x << endl;
typedef long long ll;
typedef unsigned long long ull;
typedef long double ld;
const ll maxn = 3e5 + 25;
const ll inf = 2e9 + 0;
const ll mod = 998244353;
const ll dx[] = {-1, 1, 0, 0};
const ll dy[] = {0, 0, -1, 1};
using namespace std;
ll n, m;
pair <ll, ll> p[maxn];
bool ok (ll x)
{
ll cur = 0;
for (ll i = 1; i <= n; ++i)
{
p[i].ff = max(p[i].ff, p[i].ss);
if (m * p[i].ff >= x)
{
cur += (x + p[i].ff - 1) / p[i].ff;
}
else
{
cur += m + (x - p[i].ff * m + p[i].ss - 1) / p[i].ss;
}
}
return (cur <= n * m);
}
void ma1n (/* SABR */)
{
cin >> n >> m;
for (ll i = 1; i <= n; ++i)
{
cin >> p[i].ff;
}
for (ll i = 1; i <= n; ++i)
{
cin >> p[i].ss;
}
ll l = 0, r = 1LL << 60, ans = 0;
while (l <= r)
{
ll mid = (l + r) >> 1;
if (ok(mid))
{
l = mid + 1;
ans = mid;
}
else
{
r = mid - 1;
}
}
cout << ans;
}
int main()
{
ios::sync_with_stdio(false);
cin.tie(0); cout.tie(0);
// freopen("angry.in", "r", stdin);
// freopen("angry.out", "w", stdout);
int ttt = 1;
// cin >> ttt;
for (int test = 1; test <= ttt; ++test)
{
// cout << "Case " << test << ":" << ' ';
ma1n();
}
return 0;
}
