#include <bits/stdc++.h>
using namespace std;
int n, K, v[1000050], c[1000050], ans[1000050];
double Ans, ret;
int main() {
  scanf("%d%d", &n, &K);
  for (int i = 1; i <= n; ++i) {
    scanf("%d", &v[i]);
    v[i] *= 100;
  }
  for (int i = 1; i <= n; ++i) scanf("%d", &c[i]);
  for (int i = n; i; --i) ans[i] = min(max(v[i], ans[i + 1]), c[i]);
  sort(ans + 1, ans + n + 1);
  ret = 1.0 * K / n;
  Ans = 0;
  for (int i = 1; i <= n - K + 1; ++i) {
    Ans += ans[i] * ret;
    ret = ret * (n - K - i + 1) / (n - i);
  }
  printf("%.9lf\n", Ans);
}
