#include <bits/stdc++.h>
using namespace std;
int const maxn = 1e6 + 5, logg = 20;
int v[maxn], c[maxn], good[maxn], need[maxn];
int max_v[maxn][logg], min_c[maxn][logg];
inline int get_max(int l, int r) {
  int t = need[r - l + 1];
  return max(max_v[l][t], max_v[r - (1 << t) + 1][t]);
}
inline int get_min(int l, int r) {
  int t = need[r - l + 1];
  return min(min_c[l][t], min_c[r - (1 << t) + 1][t]);
}
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  int n, k;
  cin >> n >> k;
  for (int i = 1; i <= n; ++i) {
    cin >> v[i];
    v[i] *= 100;
  }
  for (int i = 1; i <= n; ++i) cin >> c[i];
  for (int i = 1; i <= n; ++i) {
    max_v[i][0] = v[i];
    min_c[i][0] = c[i];
  }
  for (int j = 1; j < logg; ++j) {
    for (int i = 1; i + (1 << j) - 1 <= n; ++i) {
      max_v[i][j] = max(max_v[i][j - 1], max_v[i + (1 << (j - 1))][j - 1]);
      min_c[i][j] = min(min_c[i][j - 1], min_c[i + (1 << (j - 1))][j - 1]);
    }
  }
  int nxt = 2;
  for (int i = 2; i <= n; ++i) {
    need[i] = need[i - 1];
    if (i == nxt) need[i]++, nxt *= 2;
  }
  for (int i = 1; i <= n; ++i) {
    int l = i, r = n + 1;
    while (r - l > 1) {
      int m = (r + l) / 2;
      if (get_max(i, m) <= get_min(i, m))
        l = m;
      else
        r = m;
    }
    for (int pos = l; pos <= min(n, l + 1); ++pos) {
      good[i] = max(good[i], min(get_max(i, pos), get_min(i, pos)));
    }
  }
  sort(good + 1, good + n + 1);
  long double ans = 0.0, t = k / (long double)n;
  for (int i = 1; i <= n - k + 1; ++i) {
    ans += (long double)good[i] * t;
    if (i != n - k + 1)
      t = t * (long double)(n - i - k + 1) / (long double)(n - i);
  }
  cout << fixed << setprecision(18) << ans << '\n';
  return 0;
}
