#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<int,int> ii;
typedef pair<ll,ll> pll;
typedef vector<int> vi;
typedef vector<ii> vii;
typedef vector<ll> vll;
typedef vector<pll> vpll;
#define PB push_back
#define PF push_front
#define PPB pop_back
#define PPF pop_front
#define X first
#define Y second
#define MP make_pair
#define all(x) (x).begin(), (x).end()
const int mod = 1e9 + 7; //998244353;
const int inf = 1e9 + 7;
const ll INF = 1e18 + 7;
const int logo = 20;
const int MAXN = 1e6 + 7;
const int off = 1 << logo;
const int trsz = off << 1;
const int dx[] = {1, -1, 0, 0};
const int dy[] = {0, 0, -1, 1};
ll a[MAXN], b[MAXN], n, m;
bool check(ll x){
ll ret = n * m;
for(int i=1; i<=n and ret >= 0; i++){
ll uzmi = (x + a[i] - 1) / a[i];
if(uzmi <= m){
ret -= uzmi;
continue;
}
ret -= m;
ll nx = x - m * a[i];
uzmi = (nx + b[i] - 1) / b[i];
ret -= uzmi;
}
return (ret >= 0);
}
void solve(){
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 lo = 2, hi = INF, ret = 1;
while(lo <= hi){
ll mid = (lo + hi) / 2;
if(check(mid)) lo = mid + 1, ret = mid;
else hi = mid - 1;
}
cout << ret << "\n";
}
int main(){
ios_base::sync_with_stdio(false);
cin.tie(NULL);
cout.tie(NULL);
int tt = 1;
//cin >> tt;
while(tt--) solve();
return 0;
}
