#include <bits/stdc++.h>
#pragma GCC optimize("O3,unroll-loops")
#pragma GCC target("avx2")
using namespace std;
#define int long long
#define pii pair<int,int>
#define ff first
#define ss second
#define sp << " " <<
#define all(cont) cont.begin(),cont.end()
#define vi vector<int>
const int inf = 2e18,N = 5e5+1,MOD = 998244353;
void solve() {
int n,m;
cin >> n >> m;
vi a(n+1),b(n+1);
for (int i=1;i<=n;i++) cin >> a[i];
for (int i=1;i<=n;i++) cin >> b[i];
for (int i=1;i<=n;i++) a[i] = max(a[i],b[i]);
auto check = [&](int x) -> bool {
int tot = 0;
for (int i=1;i<=n;i++) {
int need = min(n*m-tot,min(m,(x+a[i]-1)/a[i]));
tot+=need;
tot+=max(0ll,(x-need*a[i]+b[i]-1))/b[i];
if (tot > n*m) return false;
}
return true;
};
int l = 0;
int r = inf;
while (l<=r) {
int x = (l+r) >> 1;
if (check(x)) l = x+1;
else r = x-1;
}
cout << r << '\n';
}
int32_t main() {
ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
#ifdef Dodi
freopen("in.txt","r",stdin);
freopen("out.txt","w",stdout);
#endif
int t = 1;
//cin >> t;
while (t --> 0) solve();
}
