#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<double, 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;
    }
    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;
    }
    sort(v.begin(), v.end());
    vector<double> prob(n), ps(n);
    ps[0] = v[0].second;
    prob[0] = (v[0].first * v[0].second);
    for(int i = 1; i < n; i++) {
        ps[i] = ps[i - 1] + v[i].second;
        prob[i] = prob[i - 1] + (v[i].first * v[i].second);
    }
    int qnt = lower_bound(v.begin(), v.end(), make_pair(ss, 0.0)) - v.begin();
    while(k--) {
        while(v[qnt].first < ss) {
            qnt++;
        }
        ss = ss * ps[qnt - 1] + (prob[n - 1] - prob[qnt - 1]);
    }
    cout << ss << "\n";
    return 0;
}