#include <bits/stdc++.h>
using namespace std;
const int N = 1000500;
int n, k, v[N], c[N], p[N];
int main(void) {
  scanf("%d%d", &n, &k);
  for (int i = 0; i < n; ++i) scanf("%d", &v[i]);
  for (int i = 0; i < n; ++i) scanf("%d", &c[i]);
  for (int i = n - 1; i >= 0; --i) p[i] = min(max(100 * v[i], p[i + 1]), c[i]);
  sort(p, p + n);
  double ans = 0, t = (double)k / n;
  for (int i = 0; n - i >= k; ++i) {
    ans += p[i] * t;
    t *= n - i - k;
    t /= n - 1 - i;
  }
  printf("%.12f\n", ans);
  return 0;
}
