#include <bits/stdc++.h>
#ifdef dbg
#define D(...) fprintf(stderr, __VA_ARGS__)
#define DD(...) D(#__VA_ARGS__ " = "), debug_helper::debug(__VA_ARGS__), D("\n")
#include "C:\Users\wsyear\Desktop\OI\templates\debug.hpp"
#else
#define D(...) ((void)0)
#define DD(...) ((void)0)
#endif
#define rep(i, j, k) for (int i = (j); i <= (k); ++i)
#define per(i, j, k) for (int i = (j); i >= (k); --i)
#define SZ(v) int((v).size())
#define ALL(v) (v).begin(),(v).end()
#define fi first
#define se second
using ll = long long;
using pii = std::pair<int, int>;
using pll = std::pair<ll, ll>;

using namespace std;

#define double long double

const double PI = acos(-1.0);

struct Complex {
  double x, y, kx, ky;

  Complex(double _x = 0.0, double _y = 0.0, double _kx = 0.0, double _ky = 0.0) {
    x = _x, y = _y, kx = _kx, ky = _ky;
  }

  Complex operator-(const Complex &b) const {
    return Complex(x - b.x, y - b.y, kx - b.kx, ky - b.ky);
  }

  Complex operator+(const Complex &b) const {
    return Complex(x + b.x, y + b.y, kx + b.kx, ky + b.ky);
  }

  Complex operator*(const Complex &b) const {
    assert(fabs(kx) < 1e-9 && fabs(ky) < 1e-9);
    return Complex(x * b.x - y * b.y, x * b.y + y * b.x, x * b.kx - y * b.ky, y * b.kx + x * b.ky);
  }
};

const int N = 100010;

int n, m, k, len, a[N];
Complex b[N], c[N];
double C[N], B[N], A[N];

double calc(double x) {
  double mx = 0;
  rep (i, 0, n - 1) mx = max(mx, x * x * A[i] + x * B[i] + C[i]);
  return mx;
}

int main() {
  cout << fixed << setprecision(10);
  cin >> n >> k;
  rep (i, 0, n - 1) cin >> a[i];
  rep (i, 0, n - 1) b[i] = Complex(a[i], 0, 0, 0);
  b[k] = Complex(0, 0, 1, 0);
  rep (i, 0, n - 1) {
    c[i] = Complex(0, 0, 0, 0);
    rep (s, 0, n - 1) {
      double coe = -2 * PI * i * s / n;
      c[i] = c[i] + Complex(cos(coe), sin(coe), 0, 0) * b[s];
    }
  }
  rep (i, 0, n - 1) {
    b[i] = c[i];
    C[i] = b[i].x * b[i].x + b[i].y * b[i].y;
    B[i] = 2. * (b[i].x * b[i].kx + b[i].y * b[i].ky);
    A[i] = b[i].kx * b[i].kx + b[i].ky * b[i].ky;
  }
  ll l = -1e15, r = 1e15;
  double ans = 1e100;
  while (r - l + 1 > 5) {
    ll mid1 = l + (r - l + 1) / 3;
    ll mid2 = r - (r - l + 1) / 3;
    double res1 = calc(mid1), res2 = calc(mid2);
    ans = min(ans, min(res1, res2));
    if (res1 <= res2) r = mid2;
    else l = mid1;
  }
  rep (i, l, r) ans = min(ans, calc(i));
  cout << sqrtl(ans) << '\n';
}