#include <bits/stdc++.h>
#define ll long long
using namespace std;
#define f first
#define s second
#define pb push_back
#define ep emplace
#define eb emplace_back
#define lb lower_bound
#define ub upper_bound
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()
#define uniquev(v) sort(all(v)), (v).resize(unique(all(v)) - (v).begin())
#define mem(f,x) memset(f , x , sizeof(f))
#define sz(x) (int)(x).size()
#define __lcm(a, b) (1ll * ((a) / __gcd((a), (b))) * (b))
#define mxx *max_element
#define mnn *min_element
#define cntbit(x) __builtin_popcountll(x)
#define len(x) (int)(x.length())
const int N = 3e5 + 10;
ll a[N], b[N];
void solve() {
ll n, m;
cin >> n >> m;
for (int i = 1; i <= n; i++) {
cin >> a[i];
}
for (int i = 1; i <= n; i++) {
cin >> b[i];
}
ll l = 0, r = 1e18, ans;
while (l <= r) {
ll mid = (l + r) / 2;
ll tot = 0;
for (int i = 1; i <= n; i++) {
if (b[i] >= a[i]) {
tot = (tot + mid / b[i] + bool(mid % b[i]));
} else {
if (a[i] * m >= mid) {
tot += (mid / a[i] + bool(mid % a[i]));
} else {
tot += m;
ll re = mid - a[i] * m;
tot += (re / b[i] + bool(re % b[i]));
}
}
if (tot > n * m)
break;
}
if (tot <= n * m) {
ans = mid;
l = mid + 1;
} else {
r = mid - 1;
}
}
cout << ans << '\n';
}
int main() {
ios_base::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
int t = 1;
//cin >> t;
while (t--) {
solve();
}
return 0;
}
