#include <bits/stdc++.h>
using namespace std;
const int MX = 1000005;
int n, k, v[MX], c[MX], l[MX], mn = 1e9, mx = 0;
long double e[MX];
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout << fixed << setprecision(10);
  cin >> n >> k;
  for (int i = 0; i < n; i++) {
    cin >> v[i];
    v[i] *= 100;
  }
  for (int i = 0; i < n; i++) cin >> c[i];
  int act;
  for (int i = n - 1; i >= 0; i--) {
    act = min(c[i], max(v[i], act));
    l[i] = act;
  }
  sort(l, l + n);
  long double res = 0, f = k;
  for (int i = 0; i < n - k + 1; i++) {
    f /= n - i;
    e[i] = f * l[i];
    f *= n - k - i;
  }
  for (int i = n - k; i >= 0; i--) res += e[i];
  cout << res << '\n';
  return 0;
}
