#include <bits/stdc++.h>
using namespace std;
const int N = 1e6 + 5;
int v[N], c[N], rr[N];
int main() {
  int n, i, j, k;
  scanf("%d%d", &n, &k);
  for (i = 0; i < n; ++i) scanf("%d", &v[i]);
  for (i = 0; i < n; ++i) scanf("%d", &c[i]);
  double ret = 0, temp = 1;
  for (i = n - 1; i >= 0; --i) rr[i] = min(max(100 * v[i], rr[i + 1]), c[i]);
  sort(rr, rr + n);
  temp *= k;
  temp /= n;
  for (i = 0; k - i > 0; ++i) {
    ret += temp * rr[i];
    if (n - i - 1 == 0) break;
    temp *= (n - k - i);
    temp /= (n - i - 1);
  }
  printf("%0.9lf\n", ret);
}
