#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
#include <bits/stdc++.h>
using namespace std;
using namespace __gnu_pbds;
typedef long long ll;
typedef long double ldb;
typedef pair<int,int> pii;
typedef pair<ll,ll> pll;
typedef pair<ldb,ldb> pdd;
#define ff(i,a,b) for(int i = a; i <= b; i++)
#define fb(i,b,a) for(int i = b; i >= a; i--)
#define trav(a,x) for(auto& a : x)
#define sz(a) (int)(a).size()
#define fi first
#define se second
#define pb push_back
#define lb lower_bound
#define ub upper_bound
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
template<typename T>
using ordered_set = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
// os.order_of_key(k) the number of elements in the os less than k
// *os.find_by_order(k) print the k-th smallest number in os(0-based)
const int mod = 1000000007;
const ll inf = 3e18 + 5;
const int mxN = 300005;
int n, m;
ll A[mxN];
ll B[mxN];
bool check(ll X){
ll uk = 0;
ff(i,1,n){
if(A[i] > B[i]){
ll a = min((ll)m, (X + A[i] - 1) / A[i]);
ll Y = max(0ll, X - 1ll * a * A[i]);
ll b = (Y + B[i] - 1) / B[i];
uk += a + b;
}
else{
ll b = (X + B[i] - 1) / B[i];
uk += b;
}
}
return uk <= 1ll * n * m;
}
int main(){
cin.tie(0)->sync_with_stdio(0);
cin >> n >> m;
ff(i,1,n)cin >> A[i];
ff(i,1,n)cin >> B[i];
ll l = 1, r = inf, ans = 0;
while(l <= r){
ll mid = l + (r - l) / 2;
if(check(mid)){
ans = mid;
l = mid + 1;
}
else r = mid - 1;
}
cout << ans << '\n';
return 0;
}
/*
3 3
19 4 5
2 6 2
2 1
9 7
2 6
5 60000
630510219 369411957 874325200 990002527 567203997
438920902 634940661 593780254 315929832 420627496
4 25
1 2 3 4
1 2 3 4
// probati bojenje sahovski
*/
