#include <bits/stdc++.h>
using namespace std;
bool dbg = 0;
void bad(string mes = "Impossible") {
  cout << mes;
  exit(0);
}
template <typename T>
string bin(T x, int st = 2) {
  string ans = "";
  while (x > 0) {
    ans += char('0' + x % st);
    x /= st;
  }
  reverse(ans.begin(), ans.end());
  return ans.empty() ? "0" : ans;
}
template <typename T>
T input() {
  T ans = 0, m = 1;
  char c = ' ';
  while (!((c >= '0' && c <= '9') || c == '-')) {
    c = getchar();
  }
  if (c == '-') m = -1, c = getchar();
  while (c >= '0' && c <= '9') {
    ans = ans * 10 + (c - '0'), c = getchar();
  }
  return ans * m;
}
template <typename T>
void read(T& a) {
  a = input<T>();
}
template <typename T>
void read(T& a, T& b) {
  read(a), read(b);
}
template <typename T>
void read(T& a, T& b, T& c) {
  read(a, b), read(c);
}
template <typename T>
void read(T& a, T& b, T& c, T& d) {
  read(a, b), read(c, d);
}
void reads(string& second) {
  string ans = "";
  char c = '\n';
  while (c == '\n' || c == ' ') c = getchar();
  while (c != '\n' && c != ' ' && c) ans += c, c = getchar();
  second = ans;
}
const int inf = 1e9;
const double eps = 1e-9;
const int maxn = 1e6 + 10, base = 1e9 + 7;
const long long llinf = 1e18 + 5;
template <typename T>
T binpow(T n, T second) {
  if (second <= 0) return 1LL;
  if (second % 2 == 0) {
    T b = binpow(n, second / 2);
    return (1LL * b * b) % base;
  } else {
    return (1LL * binpow(n, second - 1) * n) % base;
  }
}
int v[maxn], c[maxn];
int lg[maxn];
int mnSt[maxn][22];
int n, k;
void buildMn() {
  for (int i = 0; i < n; i++) mnSt[i][0] = c[i];
  for (int i = 1; i < 22; i++)
    for (int j = 0; j + (1 << i) <= n; j++) {
      mnSt[j][i] = min(mnSt[j][i - 1], mnSt[j + (1 << (i - 1))][i - 1]);
    }
}
int mxSt[maxn][22];
void buildMx() {
  for (int i = 0; i < n; i++) mxSt[i][0] = v[i];
  for (int i = 1; i < 22; i++)
    for (int j = 0; j + (1 << i) <= n; j++) {
      mxSt[j][i] = max(mxSt[j][i - 1], mxSt[j + (1 << (i - 1))][i - 1]);
    }
}
int get_min(int l, int r) {
  int j = lg[r - l + 1];
  return min(mnSt[l][j], mnSt[r - (1 << j) + 1][j]);
}
int get_max(int l, int r) {
  int j = lg[r - l + 1];
  return max(mxSt[l][j], mxSt[r - (1 << j) + 1][j]);
}
int main() {
  for (int i = 2; i < maxn; i++) {
    lg[i] = lg[i / 2] + 1;
  }
  read(n, k);
  for (int i = 0; i < n; i++) read(v[i]), v[i] *= 100;
  for (int i = 0; i < n; i++) read(c[i]);
  buildMn();
  buildMx();
  long double res = 0;
  vector<int> mda;
  for (int i = 0; i < n; i++) {
    int l = i, r = n - 1;
    while (r - l > 1) {
      int m = (l + r) >> 1;
      if (get_min(i, m) < get_max(i, m))
        r = m;
      else
        l = m;
    }
    int kek = -inf;
    for (int j = max(i, l - 5); j <= min(n - 1, l + 5); j++) {
      kek = max(kek, min(get_min(i, j), get_max(i, j)));
    }
    mda.push_back(kek);
  }
  sort((mda).begin(), (mda).end());
  long double cf = (long double)n / k;
  int first = n - 1;
  for (int i = 0; first >= k - 1; i++, first--) {
    res += (long double)mda[i] / cf;
    cf *= (long double)n - i - 1;
    cf /= (long double)n - k - i;
  }
  cout << fixed << setprecision(20) << res;
  return 0;
}
