#include <bits/stdc++.h>
using namespace std;
const double EPS = 1e-8;
const long long SZ = 1000010, SSZ = 21, APB = 26, one = 93,
                INF = 0x3fffffffffffffff, mod = 1000000007;
const int intINF = 0x3f3f3f3f;
double PI = acos(-1);
long long n, knum, a1[SZ], a2[SZ], maxv[SZ], minv[SZ];
int tim;
long long p[SZ];
void init() {
  cin >> n >> knum;
  for (long long i = 1; i <= n; ++i) cin >> a1[i], a1[i] *= 100;
  for (long long i = 1; i <= n; ++i) cin >> a2[i];
  p[n] = min(a1[n], a2[n]);
  for (long long i = n - 1; i >= 1; --i) {
    p[i] = min(max(a1[i], p[i + 1]), a2[i]);
  }
  sort(p + 1, p + 1 + n);
  double res = 0, P = 1;
  for (long long i = 1LL; i + knum - 1 <= n; ++i) {
    double pos = 1.0 * knum / (n - i + 1) * P;
    res += p[i] * pos;
    P -= pos;
  }
  cout << fixed << setprecision(10) << res << endl;
}
void work() {}
int main() {
  std::ios::sync_with_stdio(0);
  long long casenum;
  {
    init();
    work();
  }
  return 0;
}
