#include <bits/stdc++.h>
using namespace std;
inline int read() {
  int first = 0, f = 1;
  char ch = getchar();
  for (; ch < '0' || ch > '9';) {
    if (ch == '-') f = -1;
    ch = getchar();
  }
  for (; ch >= '0' && ch <= '9';) {
    first = first * 10 + ch - '0';
    ch = getchar();
  }
  return first * f;
}
int n, k;
int v[1000010], c[1000010];
int r[1000010];
int main() {
  n = read(), k = read();
  for (int i = 1; i <= n; i++) v[i] = read();
  for (int i = 1; i <= n; i++) c[i] = read();
  for (int i = n; i >= 1; i--) r[i] = min(max(100 * v[i], r[i + 1]), c[i]);
  sort(r + 1, r + n + 1);
  double ans = 0.0, tmp = 1.0;
  tmp *= 1.0 * k;
  tmp /= 1.0 * n;
  for (int i = 1; i <= k; i++) {
    ans += tmp * r[i];
    if (n - i == 0) break;
    tmp *= 1.0 * (n - k - i + 1);
    tmp /= 1.0 * (n - i);
  }
  printf("%.12lf\n", ans);
}
