#include <bits/stdc++.h>
using namespace std;
const int N = (int)1e6 + 7;
const int MOD = (int)1e9 + 7;
const long long INF = (long long)1e18 + 7;
int a[N][21], b[N][21];
int lg[N];
int n, k;
int getmn(int l, int r) {
  int k = lg[r - l + 1];
  return min(b[l][k], b[r - (1 << k) + 1][k]);
}
int getmx(int l, int r) {
  int k = lg[r - l + 1];
  return max(a[l][k], a[r - (1 << k) + 1][k]);
}
int solve(int l, int r) {
  int i = l;
  while (l + 1 < r) {
    int mid = (l + r) >> 1;
    if (getmn(i, mid) >= getmx(i, mid))
      l = mid;
    else
      r = mid;
  }
  int ans = min(getmn(i, l), getmx(i, l));
  if (l < n) ans = max(ans, min(getmn(i, l + 1), getmx(i, l + 1)));
  return ans;
}
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  cin >> n >> k;
  for (int i = 1; i <= n; i++) {
    cin >> a[i][0];
    a[i][0] *= 100;
  }
  for (int i = 1; i <= n; i++) cin >> b[i][0];
  for (int k = 1; k <= 20; k++)
    for (int i = 1; i <= n - (1 << k) + 1; i++) {
      a[i][k] = max(a[i][k - 1], a[i + (1 << (k - 1))][k - 1]);
      b[i][k] = min(b[i][k - 1], b[i + (1 << (k - 1))][k - 1]);
    }
  vector<long long> ans;
  for (int i = 2; i <= n; i++) lg[i] = lg[i / 2] + 1;
  for (int i = 1; i <= n; i++) ans.push_back(solve(i, n));
  sort(ans.begin(), ans.end());
  long double cur = 1.0 * k / (long double)n;
  long double res = 1.0 * ans[0] * cur;
  for (int i = 1; i <= n - k; i++) {
    cur *= (long double)(n - k - i + 1) / (long double)(n - i);
    res += (long double)cur * ans[i];
  }
  cout << setprecision(7) << fixed << res;
}
