#include <bits/stdc++.h>
// Author: Kazuki_Will_Win_VOI_8703
#define fi first
#define se second
#define pii pair<int, int>
#define int long long
#define all(a) a.begin(), a.end()
using namespace std;
const int mn = 3e5 + 5, inf = 1e18;
int n, m, a[mn], b[mn];
bool check(int val){
int lef = n * m;
for(int i = 1; i <= n; i++){
int Megumi = max(a[i], b[i]);
if(val <= Megumi * m) lef -= (val + Megumi - 1) / Megumi;
else{
int cur = val - Megumi * m;
lef -= m + (cur + b[i] - 1) / b[i];
}
if(lef < 0) return false;
}
return true;
}
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];
int l = 1, r = 1e18, ans = -1;
while(l <= r){
int mid = (l + r) >> 1;
if(check(mid)){
ans = mid;
l = mid + 1;
}
else r = mid - 1;
}
cout << ans << '\n';
}
signed main(){
ios_base::sync_with_stdio(false);
cin.tie(NULL);
cout.tie(NULL);
int t = 1;
// cin >> t;
while(t--) solve();
}
