#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<int, int> pll;
#define fastio ios::sync_with_stdio(false), cin.tie(0)
#pragma GCC optimize("Ofast")
#define pb push_back
#define eb emplace_back
#define f first
#define s second
#define lowbit(x) x&-x
const int maxn = 2e5 + 5;
const ll INF = 8e18;
int main(void){
fastio;
ll n, m;
cin>>n>>m;
vector<ll> a(n), b(n);
for(int i = 0; i < n; i++) cin>>a[i];
for(int i = 0; i < n; i++) cin>>b[i];
ll l = 0, r = INF;
while(l + 1 < r){
ll mid = (l + r) / 2;
vector<ll> have(n, 0);
ll nd = 0;
for(int i = 0; i < n; i++){
if(a[i] <= b[i]){
nd += (mid - 1) / b[i] + 1;
}
else if(a[i] * m >= mid){
nd += (mid - 1) / a[i] + 1;
}
else{
nd += m + (mid - a[i] * m - 1) / b[i] + 1;
}
if(nd > n * m) break;
}
if(nd > n * m) r = mid;
else l = mid;
}
cout<<l<<"\n";
}
