#include <bits/stdc++.h>
using namespace std;
#define forn(i,n) for(int i=0;i<int(n);i++)
#define forsn(i,s,n) for(int i=int(s);i<int(n);i++)
#define dforn(i,n) for(int i=int(n)-1;i>=0;i--)
#define dforsn(i,s,n) for(int i=int(n)-1;i>=int(s);i--)
#define fst first
#define snd second
#define pb push_back
#define eb emplace_back
#define sz(x) (int)x.size()
#define all(x) x.begin(), x.end()
typedef long long ll;
typedef vector<ll> vll;
typedef vector<int> vi;
typedef pair<int,int> ii;
int main(){
ios::sync_with_stdio(0);
cin.tie(0);
ll n,m;
cin>>n>>m;
vll a(n),b(n);
forn(i,n) cin>>a[i];
forn(i,n) cin>>b[i], a[i]=max(a[i],b[i]);
auto check=[&](ll k){
ll need=0;
forn(i,n){
ll curr=min((k+a[i]-1)/a[i],m);
need+=curr;
if(need>n*m) return false;
ll leftover=k-a[i]*curr;
if(leftover>=0){
need+=(leftover+b[i]-1)/b[i];
if(need>n*m) return false;
}
}
return true;
};
ll lo=0,hi=1e18;
while(hi-lo>1){
ll mid=lo+(hi-lo)/2;
if(check(mid)) lo=mid;
else hi=mid;
}
cout<<lo<<'\n';
return 0;
}
