#include <bits/stdc++.h>
#pragma GCC optimize ("Ofast", "unroll-loops")
using namespace std;
#define ll long long
#define int ll
#define FOR(i,a,b) for (int i = (a); i<(b); i++)
#define REP(i,n) FOR(i,0,n)
#define REP1(i,n) FOR(i,1,n+1)
#define RREP(i,n) for (int i=(n)-1; i>=0; i--)
#define f first
#define s second
#define pb push_back
#define ALL(x) x.begin(),x.end()
#define SZ(x) (int)(x.size())
#define SQ(x) (x)*(x)
#define pii pair<int, int>
#define pdd pair<double ,double>
#define pcc pair<char, char>
#define endl '\n'
//#define TOAD
#ifdef TOAD
#define bug(x) cerr<<__LINE__<<": "<<#x<<" is "<<x<<endl
#define IOS()
#else
#define bug(...)
#define IOS() ios::sync_with_stdio(0), cin.tie(0), cout.tie(0)
#endif
const ll inf = 1ll<<60;
const int iinf=2147483647;
const ll mod = 1e9+7;
const ll maxn=4e5+5;
const double PI=acos(-1);
ll pw(ll x, ll p, ll m=mod){
ll ret=1;
while (p>0){
if (p&1){
ret*=x;
ret%=m;
}
x*=x;
x%=m;
p>>=1;
}
return ret;
}
ll inv(ll a, ll m=mod){
return pw(a,m-2);
}
//=======================================================================================
signed main (){
IOS();
int n,m; cin>>n>>m;
vector<int> a(n), b(n);
REP(i,n){
cin>>a[i];
}
REP(i,n){
cin>>b[i];
}
int l=0, r=1e18;
while(l<=r){
//cout<<l<<' '<<r<<endl;
if (l==r){
cout<<l<<endl;
break;
}
int mid=(l+r+1)/2;
int debt=0;
REP(i,n){
if (max(a[i], b[i])*m>=mid){
int tot=mid/max(a[i], b[i])+(mid%max(a[i], b[i])>0);
debt-=m-tot;
}
else {
int mx=mid;
mx-=max(a[i], b[i])*m;
debt+=mx/b[i]+(mx%b[i]>0);
}
}
if (debt>0){
r=mid-1;
}
else l=mid;
}
}
