#include <bits/stdc++.h>
using namespace std;
struct __timestamper {};
int main() {
  int n, k;
  scanf("%d%d", &n, &k);
  vector<int> v(n);
  vector<int> c(n);
  for (int& x : v) {
    scanf("%d", &x);
    x *= 100;
  }
  for (int& x : c) {
    scanf("%d", &x);
  }
  for (int i = 0; i < n; i++) {
    v[i] = min(v[i], c[i]);
  }
  deque<int> q;
  vector<int> ans;
  for (int i = n - 1; i >= 0; i--) {
    while (q.size() && q.front() < v[i]) q.pop_front();
    q.push_front(v[i]);
    bool any = false;
    while (q.size() && q.back() > c[i]) {
      any = true;
      q.pop_back();
    }
    if (any) {
      q.push_back(c[i]);
    }
    assert(!q.empty());
    ans.push_back(q.back());
  }
  sort(ans.begin(), ans.end());
  double res = 0;
  double cur = k * 1.0 / n;
  double sumcur = 0;
  for (int i = 0; i <= n - k; i++) {
    sumcur += cur;
    res += cur * ans[i];
    cur /= (n - i - 1);
    cur *= (n - i - k);
  };
  printf("%.10f\n", res);
  return 0;
}
