#include <bits/stdc++.h>
using namespace std;
#define int long long
#define rep(i,a,b) for (int i = a; i <= b; ++i)
const int MAXN = 2e5+10;
int a[MAXN],b[MAXN];
int32_t main() {
ios_base::sync_with_stdio(0); cin.tie(nullptr);
int n,m;cin >> n >> m;
rep(i,1,n) cin >> a[i];
rep(i,1,n) cin >> b[i];
int lo = 1, hi = 1e18, ans;
while (lo <= hi) {
int mid = (lo + hi) / 2;
int cnt = n*m;
vector<int> lvl(n+1,0);
rep(i,1,n) {
if (a[i] <= b[i]) continue;
int k = min(m,(mid+a[i]-1)/a[i]);
cnt -= k;
lvl[i] += k*a[i];
}
bool ok = 1;
rep(i,1,n) {
if (lvl[i] >= mid) continue;
int k = (mid-lvl[i]+b[i]-1)/b[i];
if (cnt < k) {
ok = 0;
break;
}
cnt -= k;
}
if (ok) {
ans = mid;
lo = mid + 1;
} else {
hi = mid - 1;
}
}
cout << ans << '\n';
}
