#include <bits/stdc++.h>
#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef long double ld;
#define S second
#define F first
#define all(x) (x).begin(),(x).end()
#define sz size()
#define Yes cout << "YES" << endl
#define No cout << "NO" << endl
#define pb(x) push_back(x);
#define endl '\n'
#define fast ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
const int N=3e5+7;
ll INF=INT_MAX,mod=1e9+7;
int TT=1;
ll power(ll x, unsigned int y)
{
ll res = 1;
x = x; // % mod;
if (x == 0) return 0;
while (y > 0)
{
if (y & 1) res = (res*x) ; // % mod;
y = y>>1;
x = (x*x) ; // % mod;
}
return res;
}
ll n , m;
ll a1[N],a2[N];
pair<ll,ll> b[N];
ll cnt[N];
bool good(ll mid)
{
memset(cnt,0,sizeof cnt);
ll garbge = 0;
for(int i = 1; i <= n; i++)
{
ll cur = b[i].S;
ll mx = b[i].F;
cnt[cur] += m*mx;
if(cnt[cur]==mid) continue;
if(cnt[cur]>mid)
{
ll dif = cnt[cur]-mid;
garbge+=(dif/mx);
cnt[cur] -= (dif/mx)*mx;
}
else
{
ll dif = mid-cnt[cur];
ll o = ((dif+a2[cur]-1)/a2[cur]);
garbge-=o;
if(garbge<0) return 0;
}
}
return 1;
}
void solve()
{
cin >> n >> m;
for(int i = 1; i <= n; i++) cin >> a1[i];
for(int i = 1; i <= n; i++) cin >> a2[i];
for(int i = 1; i <= n; i++) {b[i].F = max(a1[i],a2[i]); b[i].S=i;}
sort(b+1,b+1+n,greater<pair<ll,ll> >());
ll l = 0, r = 1;
while(good(r)) r*=2;
//cout << good(18) << endl; return;
while(l+1<r)
{
ll mid = (l+r)/2;
if(good(mid)) l = mid; else r = mid;
}
cout << l << endl;
}
int main(){
//freopen("friday.in","r",stdin);
//freopen("friday.out","w",stdout);
fast;
while(TT--)
solve();
return 0;
}
