// Born_To_Laugh - Hughie Do
#include <bits/stdc++.h>
#define alle(AC) AC.begin(), AC.end()
#define fi first
#define se second
using namespace std;
typedef long long ll;
[[maybe_unused]] const ll MOD = 998244353, INF = 3e18 + 7;
const int maxn = 3e5 + 10;
int n, m;
ll a[maxn], b[maxn];
bool check(ll val){
ll cnt = 0;
vector<int> trash;
for(int i=1; i<=n; ++i){
ll num = m * max(a[i], b[i]);
if(num >= val){
ll sth = val / max(a[i], b[i]);
if(sth * max(a[i], b[i]) < val) sth++;
cnt += m - sth;
}
else trash.push_back(i);
}
for(auto &i: trash){
ll sth = (val - m * max(a[i], b[i])) / b[i];
if(sth * b[i] < val - m * max(a[i], b[i])) sth++;
cnt -= sth;
}
if(cnt >= 0) return 1;
return 0;
}
void solve(){
cin >> n >> m;
for(int i=1; i<=n; ++i) cin >> a[i];
for(int i=1; i<=n; ++i) cin >> b[i];
ll lo = 1, hi = INF, res = lo;
while(lo <= hi){
ll mid = (lo + hi) >> 1;
if(check(mid)){
res = mid;
lo = mid + 1;
}
else hi = mid - 1;
}
cout << res << '\n';
}
signed main(){
// freopen("inp.txt", "r", stdin);
// freopen("out.txt", "w", stdout);
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
solve();
}
