#include <bits/stdc++.h>
using namespace std;
using ll = long long;
const int N = 3e5+10;
ll a[N], b[N];
int main(){
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
ll n, m;
cin >> n >> m;
for(int i = 1; i <= n; ++i) cin >> a[i];
for(int i = 1; i <= n; ++i) cin >> b[i];
ll l = 0, r = 1e18, ans;
while(l <= r){
ll mid = (l+r)>>1;
ll needed = 0, extra = 0;
for(int i = 1; i <= n; ++i){
if(mid <= max(a[i], b[i])*m){
extra += m-((mid+max(a[i], b[i])-1)/max(a[i], b[i]));
}else{
needed += ((mid-m*max(a[i], b[i])+b[i]-1)/b[i]);
if(needed >= n*m) break;
}
}
if(extra >= needed) l = mid+1, ans = mid;
else r = mid-1;
}
cout << ans << '\n';
}
