#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using dbl = long double;
const ll N = 1e6 + 10;
const ll L = 20;
const ll INF = 1e9;
dbl fac[N];
dbl logcnk(ll n, ll k) { return fac[n] - fac[n - k] - fac[k]; }
ll n;
ll k;
ll l2[N];
struct rmq {
  int32_t t[L][N];
  int32_t &operator[](ll i) { return t[0][i]; }
  void build() {
    for (ll i = 1; i < L; ++i) {
      for (ll j = 0; j + (1 << i) <= N; ++j) {
        t[i][j] = min(t[i - 1][j], t[i - 1][j + (1 << (i - 1))]);
      }
    }
  }
  ll operator()(ll l, ll r) {
    if (l == r) {
      return INF;
    }
    assert(l < r);
    ll i = l2[r - l];
    return min(t[i][l], t[i][r - (1 << i)]);
  }
} v, c;
ll dp[N];
ll f(ll l, ll r) {
  if (0 <= l && l < r && r <= n) {
    return min(-100 * v(l, r), c(l, r));
  } else {
    return -INF;
  }
}
signed main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  for (ll i = 2; i < N; ++i) {
    l2[i] = l2[i / 2] + 1;
  }
  fac[0] = 0;
  fac[1] = 0;
  for (ll i = 2; i < N; ++i) {
    fac[i] = fac[i - 1] + log(i);
  }
  cin >> n >> k;
  for (ll i = 0; i < n; ++i) {
    cin >> v[i];
    v[i] *= -1;
  }
  for (ll i = 0; i < n; ++i) {
    cin >> c[i];
  }
  v.build();
  c.build();
  for (ll i = 0; i < n; ++i) {
    ll l = i, r = n + 1;
    while (r - l > 1) {
      ll m = (l + r) / 2;
      ll fi = -100 * v(i, m);
      ll se = c(i, m);
      if (fi < se) {
        l = m;
      } else {
        r = m;
      }
    }
    dp[i] = max(f(i, l), f(i, r));
  }
  sort(dp, dp + n);
  dbl ans = 0;
  dbl kek = 0;
  for (ll i = 0; i < n - k + 1; ++i) {
    dbl prob = exp(logcnk(n - i - 1, k - 1) - logcnk(n, k));
    ans += prob * dp[i];
    kek += prob;
  }
  cout << fixed << setprecision(15) << ans << "\n";
  return 0;
}
