// luogu-judger-enable-o2
#include<cstdio>
#include<cmath>
#include<ctime>
#include<cstdlib>
#include<algorithm>
#include<cstring>
#define sqr(x) ((x)*(x))
const int MAXN = 31;
const double eps = 1e-15;
const int INF = 1e9 + 10;
using namespace std;
inline int read() {char c = getchar();int x = 0, f = 1;while(c < '0' || c > '9') {if(c == '-') f = -1;c = getchar();}while(c >= '0' && c <= '9') {x = x * 10 + c - '0';c = getchar();}return x * f;
}
int N, M;
int belong[MAXN], a[MAXN];
double sum[MAXN], Aver = 0, Best = 1e20;
void MoNiTuiHuo() {memset(sum, 0, sizeof(sum));const double DeltaT = 0.99;double ans = 0;for(int i = 1; i <= N; i++) belong[i] = rand() % M + 1, sum[ belong[i] ] += a[i];for(int i = 1; i <= M; i++) ans += sqr(sum[i] - Aver);for(double T = 10000; T > eps; T *= DeltaT) {int P = min_element(sum + 1, sum + M + 1) - sum;//找出最小的位置 int X = rand() % N + 1;//这里直接随机就可以 double Pre = ans;ans -= sqr(sum[ belong[X] ] - Aver) + sqr(sum[P] - Aver);sum[ belong[X] ] -= a[X]; sum[P] += a[X];ans += sqr(sum[ belong[X] ] - Aver) + sqr(sum[P] - Aver);        if((ans < Pre) || (exp( (ans-Pre)/T ) * RAND_MAX  < rand() )) belong[X] = P;//以一定概率接受最优解 else ans = Pre, sum[ belong[X] ] += a[X], sum[P] -= a[X];    //不更新
    }if(ans < Best) Best = ans;
}
int main() {#ifdef WIN32freopen("a.in", "r", stdin);#endifsrand(19260817);N = read(); M = read();for(int i = 1; i <= N; i++) a[i] = read(), Aver += a[i];Aver /= M;for(int i = 1; i <= 1000; i++) MoNiTuiHuo();printf("%.2lf",sqrt(Best / M));//因为y=sqrt(x)这个函数具有单调性，所以最后在开根就可以 return 0;
}