//https://codeforces.com/gym/102986/problem/F
#include <bits/stdc++.h>

using namespace std;

int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(0);
    int n, k;
    cin >> n >> k;
    vector<pair<int, long double>> v(n);
    for(int i = 0; i < n; i++) {
        cin >> v[i].first;
    }
    for(int i = 0; i < n; i++) {
        cin >> v[i].second;
    }
    long double ss = 0;
    for(int i = 0; i < n; i++) {
        ss += v[i].first * v[i].second;
    }
    if(k == 0) {
        cout << ss << "\n";
        return 0;
    }
    //k--;
    sort(v.begin(), v.end());
    vector<int> bruh;
    vector<long double> prob, ps;
    bruh.push_back(v[0].first);
    ps.push_back(v[0].second);
    prob.push_back(v[0].first * v[0].second);
    for(int i = 1; i < n; i++) {
        ps.push_back(ps[i - 1] + v[i].second);
        prob.push_back(prob[i - 1] + (v[i].first * v[i].second));
        bruh.push_back(v[i].first);
    }
    while(k--) {
        long double _ = 0;
        int qnt = lower_bound(bruh.begin(), bruh.end(), ss) - bruh.begin();
        _ += ss * ps[qnt - 1] + (prob[n - 1] - prob[qnt - 1]);
        ss = _;
    }
    cout << ss << "\n";
    return 0;
}