#include<bits/stdc++.h>
using namespace std;
#define fast ios_base::sync_with_stdio(false); cin.tie(NULL); cout.tie(NULL);
#define ll long long int
const ll N = 3e5+5 , INF = 1e18 , MOD = 1e9+7;
vector<ll> a(N),b(N);
ll n,m;
bool check(ll x){
ll cnt = 0;
for(ll i = 1; i <= n; i++){
ll maxi = max(a[i],b[i]);
if(maxi*m < x){
cnt += m;
ll cur = x-(maxi*m);
cnt += ((cur+b[i]-1)/b[i]);
}
else cnt += ((x+maxi-1)/maxi);
if(cnt > (n*m)) return false;
}
return (cnt <= (n*m));
}
void solve(){
cin >> n >> m;
for(ll i = 1; i <= n; i++) cin >> a[i];
for(ll i = 1; i <= n; i++) cin >> b[i];
ll low = 1 , high = 1e18;
ll ans;
while(low <= high){
ll mid = (low+high)/2;
if(check(mid)){
ans = mid;
low = mid+1;
}
else high = mid-1;
}
cout << ans;
}
int main(){
fast;
ll tc = 1;
// cin >> tc;
while(tc--) solve();
return 0;
}
