#include <bits/stdc++.h>
using namespace std;
//#pragma GCC optimize("Ofast,unroll-loops")
//#pragma GCC target("avx2,popcnt,lzcnt,abm,bmi,bmi2,fma,tune=native")
using ll = long long;
using ld = long double;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
using pi3 = pair<pii, int>;
#define IOS ios_base::sync_with_stdio(false); cin.tie(0); cout.tie(0);
#define F first
#define S second
#define sz(x) x.size()
#define all(x) x.begin(), x.end()
#define pb push_back
#define cl clear
#define minr(a, b) a = min(a, b);
#define maxr(a, b) a = max(a, b);
#define shit cout << "shit\n" << flush;
#define tl while(1&1) continue;
#define FOR(i, st, nd) for(int i = st; i <= nd; i++)
#define rand(l, r) uniform_int_distribution<int64_t>(l,r)(rng)
random_device device; default_random_engine rng(device());
const int Mod = 1e9 + 7; //998244353;
const int LG = 64;
const int SQ = 500;
const int Inf = 2e9 + 10;
const int maxN = 3e5 + 10;
int n, m;
ll a[maxN];
ll b[maxN];
bool check(ll x) {
ll need = 0;
for(int i = 1; i <= n; i++) {
if(a[i]*m >= x) {
need += (x+a[i]-1)/a[i];
}
else {
ll rem = x - a[i]*m;
need += m;
need += (rem+b[i]-1)/b[i];
}
}
return (need <= 1LL*n*m);
}
int main() {
IOS;
cin >> n >> m;
for(int i = 1; i <= n; i++)
cin >> a[i];
for(int i = 1; i <= n; i++) {
cin >> b[i];
a[i] = max(a[i], b[i]);
}
ll l = 0;
ll r = 1e18+1;
while(l+1 < r) {
ll mid = (l+r)>>1;
if(check(mid))
l = mid;
else
r = mid;
}
cout << l;
}
