#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define int ll
using P = pair<int, int>;
#define all(x) x.begin(), x.end()
#define rep(x,s,e) for (auto x=(s)-((s)>(e));x!=(e)-((s)>(e));((s)<(e)?x++:x--))
#define sz(x) (int)x.size()
const char nl = '\n';
const int mod = 998244353;
void solve() {
int n, m; cin >> n >> m;
vector<int> a(n), b(n);
for (auto &i: a)cin >> i;
for (auto &i: b)cin >> i;
int l = 0, r = 1e18+10;
while (r-l>1) {
int mid = l+r>>1;
int sm = 0;
rep(i, 0, n) {
int mx = max(a[i], b[i]);
//assert(mx > 0);
int x = (mid+mx-1)/mx;
if (x <= m)sm += m-x;
else {
x = mid-m*mx;
sm -= (x+b[i]-1)/b[i];
}
}
if (0 <= sm)l = mid;
else r = mid;
}
cout << l << nl;
}
int32_t main() {
ios_base::sync_with_stdio(0);
cin.tie(0);
solve();
return 0;
}
