#include<bits/stdc++.h>
using namespace std;
#define S second
#define F first
#define ll long long
//#pragma GCC optimize("Ofast, unroll-loop")
//#pragma GCC target("avx,avx2")
#pragma GCC optimize("O3")
const int inf=0x3f3f3f3f;
const ll inff=0x3f3f3f3f3f3f3f3f;
const int X=998244353;
int a[300005], b[300005];
ll divv(ll a, ll b){ // a/b
return (a+b-1)/b;
}
signed main(){
ios::sync_with_stdio(0), cin.tie(0);
int n; ll m; cin >> n >> m;
for(int i=1 ; i<=n ; i++) cin >> a[i];
for(int i=1 ; i<=n ; i++) cin >> b[i];
for(int i=1 ; i<=n ; i++) a[i]=max(a[i],b[i]);
auto ok=[&](ll goal)->bool{
ll t=0;
for(int i=1 ; i<=n ; i++){
ll c=divv(goal,a[i]);
if(c<m) t+=m-c;
else{
ll req=goal-m*a[i];
t-=divv(req,b[i]);
}
//cout << c << ' ' << t << '\n';
}
return t>=0;
};
ll ans=0;
for(ll jump=300'000'000'000'000LL ; jump ; jump>>=1){
while(ans+jump<=inff&&ok(ans+jump)) ans+=jump;
}
cout << ans << '\n';
return 0;
}
