#pragma GCC optimize ("Ofast")
#include<bits/stdc++.h>
using namespace std;
inline int my_getchar_unlocked(){
  static char buf[1048576];
  static int s = 1048576;
  static int e = 1048576;
  if(s == e && e == 1048576){
    e = fread_unlocked(buf, 1, 1048576, stdin);
    s = 0;
  }
  if(s == e){
    return EOF;
  }
  return buf[s++];
}
inline void rd(int &x){
  int k;
  int m=0;
  x=0;
  for(;;){
    k = my_getchar_unlocked();
    if(k=='-'){
      m=1;
      break;
    }
    if('0'<=k&&k<='9'){
      x=k-'0';
      break;
    }
  }
  for(;;){
    k = my_getchar_unlocked();
    if(k<'0'||k>'9'){
      break;
    }
    x=x*10+k-'0';
  }
  if(m){
    x=-x;
  }
}
struct MY_WRITER{
  char buf[1048576];
  int s;
  int e;
  MY_WRITER(){
    s = 0;
    e = 1048576;
  }
  ~MY_WRITER(){
    if(s){
      fwrite_unlocked(buf, 1, s, stdout);
    }
  }
}
;
MY_WRITER MY_WRITER_VAR;
void my_putchar_unlocked(int a){
  if(MY_WRITER_VAR.s == MY_WRITER_VAR.e){
    fwrite_unlocked(MY_WRITER_VAR.buf, 1, MY_WRITER_VAR.s, stdout);
    MY_WRITER_VAR.s = 0;
  }
  MY_WRITER_VAR.buf[MY_WRITER_VAR.s++] = a;
}
inline void wt_L(char a){
  my_putchar_unlocked(a);
}
inline void wt_L(int x){
  int s=0;
  int m=0;
  char f[10];
  if(x<0){
    m=1;
    x=-x;
  }
  while(x){
    f[s++]=x%10;
    x/=10;
  }
  if(!s){
    f[s++]=0;
  }
  if(m){
    my_putchar_unlocked('-');
  }
  while(s--){
    my_putchar_unlocked(f[s]+'0');
  }
}
inline void wt_L(long long x){
  int s=0;
  int m=0;
  char f[20];
  if(x<0){
    m=1;
    x=-x;
  }
  while(x){
    f[s++]=x%10;
    x/=10;
  }
  if(!s){
    f[s++]=0;
  }
  if(m){
    my_putchar_unlocked('-');
  }
  while(s--){
    my_putchar_unlocked(f[s]+'0');
  }
}
template<class T> inline T pow2_L(T a){
  return a*a;
}
int N;
int X[100000];
int M;
int main(){
  int i;
  double m;
  double ss = 0;
  double res;
  long long resi;
  rd(N);
  {
    int Lj4PdHRW;
    for(Lj4PdHRW=(0);Lj4PdHRW<(N);Lj4PdHRW++){
      rd(X[Lj4PdHRW]);
    }
  }
  rd(M);M += (-1);
  {
    int e98WHCEY;
    int cTE1_r3A;
    if(N==0){
      cTE1_r3A = 0;
    }
    else{
      cTE1_r3A = X[0];
      for(e98WHCEY=(1);e98WHCEY<(N);e98WHCEY++){
        cTE1_r3A += X[e98WHCEY];
      }
    }
    m =cTE1_r3A/ (double)N;
  }
  for(i=(0);i<(N);i++){
    ss +=(pow2_L((X[i] - m)));
  }
  if(ss==0){
    wt_L(50);
    wt_L('\n');
    return 0;
  }
  res = 50 + 10 * (X[M] - m) / sqrt(ss/N);
  resi = res;
  wt_L(resi);
  wt_L('\n');
  return 0;
}
// cLay varsion 20200509-1

// --- original code ---
// int N, X[1d5], M;
// {
//   double m, ss = 0, res;
//   ll resi;
//   rd(N,X(N),M--);
//   m = sum(X(N)) / (double)N;
//   rep(i,N) ss += (X[i] - m) ** 2;
//   if(ss==0) wt(50), return 0;
//   res = 50 + 10 * (X[M] - m) / sqrt(ss/N);
//   resi = res;
//   wt(resi);
// }