#include "bits/stdc++.h"
 
using namespace std;
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<string> VS;
typedef pair<int, int> PII;
typedef vector<PII> VPII;
typedef long long LL;
typedef priority_queue<int> PQ_DESC;
typedef priority_queue<int, vector<int>, greater<int>> PQ_ASC;
typedef priority_queue<PII> PQ_DESC_PII;
typedef priority_queue<PII, vector<PII>, greater<PII>> PQ_ASC_PII;

typedef vector<LL> VLL;
 
#define SORT_ASC(c) sort((c).begin(), (c).end())
#define SORT_DESC(c) sort((c).begin(), (c).end(), greater<typeof((c).begin())>())
 
#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n)  FOR(i,0,n)
#define SIZE(a) int((a).size())
 
const double EPS = 1e-10;
const double PI  = acos(-1.0);
const int INT_LARGE = 1000000100;
 
 
//debug func
template<typename T>
void vecprint(vector<T> v) {
    for(auto x : v) {
        cerr << x << " ";
    }
    cerr << endl;
}
 
template<typename T>
void vecvecprint(vector<vector<T>> v) {
    for(auto x :v) {
        REP(i, x.size()-1) {
            cerr << x[i] << " ";
        }
        cerr << x[x.size()-1] <<endl;
    }
}
 
template<typename T>
void pqprint(priority_queue<T> q) {
    while(!q.empty()) {
        cerr << q.top() << " ";
        q.pop();
    }
    cerr << endl;
}
 
template<typename T>
void qprint(queue<T> q) {
    while(!q.empty()) {
        cerr << q.front() << " ";
        q.pop();
    }
    cerr << endl;
}
 
template<typename T>
void vecqprint(vector<queue<T>> v) {
    for(int i = 0; i < v.size(); i++) {
        qprint(v[i]);
    }
}
 
template <typename Iterator>
inline bool next_combination(const Iterator first, Iterator k, const Iterator last)
{
   /* Credits: Thomas Draper */
   if ((first == last) || (first == k) || (last == k))
      return false;
   Iterator itr1 = first;
   Iterator itr2 = last;
   ++itr1;
   if (last == itr1)
      return false;
   itr1 = last;
   --itr1;
   itr1 = k;
   --itr2;
   while (first != itr1)
   {
      if (*--itr1 < *itr2)
      {
         Iterator j = k;
         while (!(*itr1 < *j)) ++j;
         iter_swap(itr1,j);
         ++itr1;
         ++j;
         itr2 = k;
         rotate(itr1,j,last);
         while (last != j)
         {
            ++j;
            ++itr2;
         }
         rotate(k,itr2,last);
         return true;
      }
   }
   rotate(first,k,last);
   return false;
}
 
LL n, k;
VLL va, vf;

int main(void) {
    cin >> n >> k;

    priority_queue<LL> pqa;
    priority_queue<LL, vector<LL>, greater<LL>> pqf;

    REP(i, n) {
        LL tmp;
        cin >> tmp;
        pqa.push(tmp);
    }
    REP(i, n) {
        LL tmp;
        cin >> tmp;
        pqf.push(tmp);
    }

    va = VLL(n, 0); vf = VLL(n, 0);
    VLL vt(n, 0);
    LL tmax = 0;
    REP(i, n) {
        va[i] = pqa.top();
        vf[i] = pqf.top();
        pqa.pop();
        pqf.pop();
        vt[i] = va[i] * vf[i];
        if(vt[i] > tmax) tmax = vt[i];
    }

    // vecprint(va);
    // vecprint(vf);
    // cerr << tmax << endl;
    LL tcur;
    LL tmin = 0;

    while(true) {
        tcur = tmin + (tmax - tmin) / 2;
        // cerr << "t: " << tcur << ", max: " << tmax << ", min:" << tmin << endl;
        if(tmax == tmin) break;
        LL kcnt = 0;
        bool flg = false;
        REP(i, n) {
            if(vt[i] > tcur) kcnt += 1 + (vt[i] - tcur - 1) / vf[i];
            if(kcnt > k) {
                flg = true;
                break;
            }
        }
        if(flg) {
            tmin = tcur + 1;
        } else {
            tmax = tcur;
        }
    }
    cout << tmax << endl;
}

Measured memory_bytes: 8176