#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<int,int> pii;

const int N=2003;
const double pi=3.141592653589793;
int n,k,a[N];
struct vec{
    long double x,y;
    vec(){}
    vec(double x,double y):x(x),y(y){}
    vec operator+(const vec& p)const{
        return vec(x+p.x,y+p.y);
    }
    vec operator-(const vec& p)const{
        return vec(x-p.x,y-p.y);
    }
    vec operator*(const double& v)const{
        return vec(x*v,y*v);
    }
}f[N];

bool chk(long double x){
    long double l=-1e9,r=1e9;
    for(int i=0;i<n;++i){
        if(x<fabs(f[i].y))return 0;
        l=max(l,f[i].x-sqrt(x*x-f[i].y*f[i].y));
        r=min(r,f[i].x+sqrt(x*x-f[i].y*f[i].y));
    }
    return floor(l)<floor(r);
}

int main(){
    ios::sync_with_stdio(0);cin.tie(0);
    cin>>n>>k;
    for(int i=0;i<n;++i)cin>>a[(i-k+n)%n];
    for(int i=0;i<n;++i)
        for(int j=1;j<n;++j)
            f[i]=f[i]+vec(a[j]*cos(-2*pi*i*j/n),a[j]*sin(-2*pi*i*j/n));
    long double l=0,r=4e6;
    while(r-l>1e-12){
        long double mid=(l+r)/2;
        if(chk(mid))r=mid;
        else l=mid;
    }
    cout<<fixed<<setprecision(12)<<l<<'\n';
}