#include <bits/stdc++.h>
using namespace std;
int i, j, k, l, n, m, s, sum;
double an;
const int N = 101000;
class arr {
 public:
  double x, y;
  int num;
} a[N];
bool com(const arr &o, const arr &p) { return o.x < p.x; }
double sqr(double x) { return x * x; }
double dis(arr a, arr b) { return sqrt(sqr(a.x - b.x) + sqr(a.y - b.y)); }
int main() {
  scanf("%d%d", &n, &k);
  for (int i = 1; i <= n; i++) {
    scanf("%lf", &a[i].x);
    a[i].num = i;
    a[i].y = 0;
  }
  sort(&a[1], &a[n + 1], com);
  scanf("%lf%lf", &a[n + 1].x, &a[n + 1].y);
  for (int i = 1; i <= n; i++)
    if (k == a[i].num) {
      k = i;
      break;
    }
  an = 999999999;
  if (n == 1) {
    an = dis(a[1], a[n + 1]);
  } else if (k == n + 1) {
    an = min(dis(a[n + 1], a[1]), dis(a[n + 1], a[n])) + dis(a[n], a[1]);
  } else {
    double s;
    s = dis(a[k], a[1]) * 2;
    for (int i = k; i < n; i++) {
      an = min(an, s + dis(a[i], a[n + 1]) + dis(a[i + 1], a[n + 1]) +
                       dis(a[n], a[i + 1]));
      an = min(an, s + dis(a[i], a[n + 1]) + dis(a[n], a[n + 1]) +
                       dis(a[n], a[i + 1]));
      an = min(an, s + dis(a[i], a[n]) + dis(a[n], a[n + 1]));
      if (i + 1 <= n) s += dis(a[i], a[i + 1]);
    }
    s = dis(a[k], a[n]) * 2;
    for (int i = k; i >= 2; i--) {
      an = min(an, s + dis(a[i], a[n + 1]) + dis(a[i - 1], a[n + 1]) +
                       dis(a[1], a[i - 1]));
      an = min(an, s + dis(a[i], a[n + 1]) + dis(a[n + 1], a[1]) +
                       dis(a[1], a[i - 1]));
      an = min(an, s + dis(a[i], a[1]) + dis(a[n + 1], a[1]));
      if (i - 1 >= 0) s += dis(a[i], a[i - 1]);
    }
    s = 0;
    for (int i = k; i >= 1; i--) {
      an = min(an, s + dis(a[i], a[n]) + dis(a[n], a[n + 1]) +
                       dis(a[n + 1], a[i - 1]) + dis(a[1], a[i - 1]));
      an = min(an, s + dis(a[i], a[n]) + dis(a[n], a[n + 1]) +
                       dis(a[n + 1], a[1]) + dis(a[1], a[i - 1]));
      an = min(an, s + dis(a[i], a[1]) + dis(a[1], a[n + 1]) +
                       dis(a[n + 1], a[n]) + dis(a[n], a[i + 1]));
      an = min(an, s + dis(a[i], a[1]) + dis(a[1], a[n + 1]) +
                       dis(a[n + 1], a[i + 1]) + dis(a[n], a[i + 1]));
      if (i - 1 > 0) s += dis(a[i], a[i - 1]);
    }
    s = 0;
    for (int i = k; i <= n; i++) {
      an = min(an, s + dis(a[i], a[1]) + dis(a[1], a[n + 1]) +
                       dis(a[n + 1], a[i + 1]) + dis(a[n], a[i + 1]));
      an = min(an, s + dis(a[i], a[1]) + dis(a[1], a[n + 1]) +
                       dis(a[n + 1], a[n]) + dis(a[n], a[i + 1]));
      an = min(an, s + dis(a[i], a[n]) + dis(a[n], a[n + 1]) +
                       dis(a[n + 1], a[1]) + dis(a[1], a[i - 1]));
      an = min(an, s + dis(a[i], a[n]) + dis(a[n], a[n + 1]) +
                       dis(a[n + 1], a[i - 1]) + dis(a[1], a[i - 1]));
      if (i + 1 <= n) s += dis(a[i + 1], a[i]);
    }
  }
  printf("%.8lf", an);
  return 0;
}
