#include <bits/stdc++.h>
 
using namespace std;
 
#define MOD 1000000007
#define mxn (int) 3e5+5
#define mxm (int) 1e5+5
#define f first
#define s second
#define pb push_back
#define es " "
#define endl "\n"
#define INF 0x3f3f3f3f
#define INFL 0x3f3f3f3f3f3f3f3f
#define fastio ios_base::sync_with_stdio(false), cin.tie(nullptr)
#define sz(x) (int)x.size()
#define all(x) x.begin(), x.end()
typedef long long ll;
typedef pair<int, int> pii;
typedef pair<pii, int> pip;
typedef unsigned long long ull;

ll n, k, a[mxn], b[mxn];

ll solve(ll x){
    ll cont=0;
    for(int i=1; i<=n; i++){
        if(b[i]>=a[i]){
            cont+=(b[i]+x-1)/b[i];
        }else{
            ll qnt = min(k, (a[i]+x-1)/a[i]);
            cont+=qnt;

            ll resta = x - a[i]*qnt;
            if(resta>0) cont+=(b[i]+resta-1)/b[i];
        }
        if(cont>(n*k)) break;
    }
    return cont<=(n*k);
}

int main(){
    fastio;
    cin >> n >> k;
    for(int i=1; i<=n; i++){
        cin >> a[i];
    }
    for(int i=1; i<=n; i++){
        cin >> b[i];
    }
    ll ini=0, fim=2e18, meio, resp;
    while(ini<=fim){
        meio=(ini+fim)/2;
        if(solve(meio)){
            ini=meio+1;
            resp=meio;
        }else{
            fim=meio-1;
        }
    }
    cout << resp << endl;
    return 0;
}