#include<iostream>
#include<cstdio>
#include<algorithm>
#include<cstring>
#include<cmath>
using namespace std;
typedef long long ll;
typedef double db;
inline int read()
{
    int x=0,f=1; char ch=getchar();
    while(ch<'0'||ch>'9') { if(ch=='-') f=-1; ch=getchar(); }
    while(ch>='0'&&ch<='9') { x=(x<<1)+(x<<3)+(ch^48); ch=getchar(); }
    return x*f;
}
const int N=11,INF=1e9;
int n,m,K,sum[N][N];
db f[N][N][N][N][N],P;
bool vis[N][N][N][N][N];
inline db calc(int xa,int ya,int xb,int yb) { return sum[xb][yb]-sum[xa-1][yb]-sum[xb][ya-1]+sum[xa-1][ya-1]; }
db dfs(int xa,int ya,int xb,int yb,int k)
{
    if(xb-xa+yb-ya<k) return INF;
    db &T=f[xa][ya][xb][yb][k];
    if(vis[xa][ya][xb][yb][k]) return T;
    vis[xa][ya][xb][yb][k]=1; T=INF;
    if(!k) { T=(calc(xa,ya,xb,yb)-P)*(calc(xa,ya,xb,yb)-P); return T; }
    for(int i=0;i<k;i++)
        for(int j=xa;j<xb;j++)
            T=min(T, dfs(xa,ya,j,yb,i)+dfs(j+1,ya,xb,yb,k-i-1) );
    for(int i=0;i<k;i++)
        for(int j=ya;j<yb;j++)
            T=min(T, dfs(xa,ya,xb,j,i)+dfs(xa,j+1,xb,yb,k-i-1) );
    return T;
}
int main()
{
    n=read(),m=read(),K=read();
    for(int i=1;i<=n;i++)
        for(int j=1;j<=m;j++) sum[i][j]=sum[i-1][j]+sum[i][j-1]-sum[i-1][j-1]+read();
    P=1.0*sum[n][m]/K;
    printf("%.2lf\n",sqrt( dfs(1,1,n,m,K-1)/K ));
    return 0;
}