#include<cstdio>

#include<cstring>

#include<algorithm>

#include<queue>

using namespace std;

int rdn()

{

  int ret=;bool fx=;char ch=getchar();

  while(ch>''||ch<''){if(ch=='-')fx=;ch=getchar();}

  while(ch>=''&&ch<='')ret=ret*+ch-'',ch=getchar();

  return fx?ret:-ret;

}

int Mn(int a,int b){return a<b?a:b;}

const int N=,M=,INF=;

int n,m,t,hd[N],xnt=,cur[N],to[M],nxt[M],cap[M],w[M];

int dis[N],pre[N],info[N],ans; bool ins[N];

queue<int> q;

void add(int x,int y,int z)

{

  to[++xnt]=y;nxt[xnt]=hd[x];hd[x]=xnt;cap[xnt]=;w[xnt]=z;

  to[++xnt]=x;nxt[xnt]=hd[y];hd[y]=xnt;cap[xnt]=;w[xnt]=-z;

}

bool spfa()

{

  memset(dis,0x3f,sizeof dis);dis[]=;

  info[]=INF;info[t]=;

  q.push();ins[]=;

  while(q.size())

    {

      int k=q.front();q.pop();ins[k]=;

      for(int i=hd[k],v;i;i=nxt[i])

    if(cap[i]&&dis[v=to[i]]>dis[k]+w[i])

      {

        dis[v]=dis[k]+w[i];

        pre[v]=i; info[v]=Mn(info[k],cap[i]);

        if(!ins[v])q.push(v),ins[v]=;

      }

    }

  return info[t];

}

void ek()

{

  int s=info[t];

  for(int i=pre[t];i;i=pre[to[i^]])

    {

      ans+=s*w[i];cap[i]-=s;cap[i^]+=s;

    }

}

int main()

{

  scanf("%d%d",&m,&n);int ct=n*m; t=ct+n+;

  for(int i=,x=ct+,d;i<=n;i++,x++)

    for(int j=,y=;j<=m;j++,y++)

      {

    d=rdn();

    for(int k=,lj=d;k<=n;k++,lj+=d,y++)

      add(x,y,lj);

      }

  for(int i=,x=ct+;i<=n;i++,x++)add(,x,);

  for(int i=;i<=ct;i++)add(i,t,);

  while(spfa())ek();

  printf("%.2f\n",(double)ans/n);

  return ;

}