#include <bits/stdc++.h>
using namespace std;
int v[1000002], c[1000002], dp[1000002];
double p[1000002];
int main() {
  int n, m, i, j, k;
  double ans;
  scanf("%d%d", &n, &k);
  for (i = 1; i <= n; i++) {
    scanf("%d", &v[i]);
    v[i] *= 100;
  }
  for (i = 1; i <= n; i++) scanf("%d", &c[i]);
  p[n - 1] = 1.0 * k / n;
  k--;
  for (i = n - 2; i >= k; i--) p[i] = (i + 1 - k) * p[i + 1] / (i + 1);
  for (i = n; i > 0; i--) dp[i] = min(c[i], max(v[i], dp[i + 1]));
  sort(dp, dp + n + 1);
  for (i = 1, ans = 0; i <= n; i++) ans += (p[n - i] * dp[i]);
  printf("%.15lf\n", ans);
  return 0;
}
