#include <bits/stdc++.h>
using namespace std;
const int N = 1e6 + 10;
int a[N], b[N], c[N];
int main() {
  int n, k;
  scanf("%d%d", &n, &k);
  for (int i = 1; i <= n; ++i) {
    scanf("%d", &a[i]);
    a[i] *= 100;
  }
  for (int i = 1; i <= n; ++i) scanf("%d", &b[i]);
  c[n] = min(a[n], b[n]);
  for (int i = n - 1; i >= 1; --i) c[i] = min(max(c[i + 1], a[i]), b[i]);
  sort(c + 1, c + n + 1);
  double P = 0, ans = 0;
  for (int i = 1; i + k - 1 <= n; ++i) {
    double p = (1 - P) * k / (n - i + 1);
    ans += c[i] * p;
    P += p;
  }
  printf("%.6f\n", ans);
  return 0;
}
