#include <bits/stdc++.h>
#define ll long long
#define ld long double
#define pb push_back
#define fi first
#define se second
#define en '\n'
#define sp ' '
#define tb '\t'
#define ri(n) int n; cin >> n
#define rl(n) ll n; cin >> n
#define rs(s) string s; cin >> s
#define rc(c) char c; cin >> c
#define rv(v) for (auto &x : v) cin >> x
#define pven(v) for (auto x : v) cout << x << en
#define pv(v) for (auto x : v) cout << x << sp; cout << en
#define all(v) v.begin(), v.end()
#define rall(v) v.rbegin(), v.rend()
#define yes cout << "YES" << en
#define no cout << "NO" << en
#define smin(a, b) a = min(a, b)
#define smax(a, b) a = max(a, b)
#define ssort(a, b) if (a < b) swap(a, b)
#define bitcnt(a) (__builtin_popcountll(a))
#define bithigh(a) (63-__builtin_clzll(a))
#define lg bithigh
#define highpow(a) (1LL << (ll)lg(a))
using namespace std;
const ll LINF = 1e18;
const int mxN = 1e6+10, INF = 2e9;
ll n, m, a[mxN], b[mxN];
bool Check(ll k){
ll cnt = 0;
for (int i = 0; i < n; i++){
ll cur = k;
if (a[i] > b[i]){
ll x = (k + a[i] - 1)/a[i];
smin(x, m);
cur -= min(cur, x*a[i]);
cnt += x;
}
ll x = (cur + b[i] - 1)/b[i];
cnt += x;
}
return cnt <= n*m;
}
void Solve(){
cin >> n >> m;
for (int i = 0; i < n; i++)
cin >> a[i];
for (int i = 0; i < n; i++)
cin >> b[i];
ll l = 1, r = LINF, ans = 0;
while (l <= r){
ll k = (l + r + 1)>>1;
if (Check(k)){
ans = k;
l = k + 1;
}
else r = k - 1;
}
cout << ans << en;
}
int main(){
ios::sync_with_stdio(false);
cin.tie(0); cout.tie(0); cerr.tie(0);
cout << setprecision(12) << fixed;
cerr << setprecision(12) << fixed;
cerr << "Started!" << endl;
int t = 1;
//cin >> t;
while (t--)
Solve();
return 0;
}
