#include <iostream>
#include <cstdlib>
#include <cstring>
#include <algorithm>
#include <cmath>
#include <cstdio>using namespace std;const int MaxN = 15 + 5, MaxT = 15 + 5;int n, m, t, Num;
int Sum[MaxN][MaxN];typedef double DB;const DB INF = 999999999;DB Ave;
DB f[MaxN][MaxN][MaxN][MaxN][MaxT];DB Get(int x, int y, int xx, int yy) {return (DB)(Sum[xx][yy] - Sum[x - 1][yy] - Sum[xx][y - 1] + Sum[x - 1][y - 1]);
}inline DB Sqr(DB x) {return x * x;}
inline DB gmin(DB a, DB b) {return a < b ? a : b;}DB Solve(int x, int xx, int y, int yy, int k) {if (f[x][xx][y][yy][k] != -1) return f[x][xx][y][yy][k];if (k == 1) return f[x][xx][y][yy][k] = Sqr(Get(x, y, xx, yy) - Ave);DB ret = INF;for (int i = x; i <= xx - 1; ++i)for (int j = 1; j <= k - 1; ++j)ret = gmin(ret, Solve(x, i, y, yy, j) + Solve(i + 1, xx, y, yy, k - j));for (int i = y; i <= yy - 1; ++i) for (int j = 1; j <= k - 1; ++j) ret = gmin(ret, Solve(x, xx, y, i, j) + Solve(x, xx, i + 1, yy, k - j));return f[x][xx][y][yy][k] = ret;
}int main()
{scanf("%d%d%d", &n, &m, &t);for (int i = 1; i <= n; ++i)for (int j = i; j <= n; ++j)for (int p = 1; p <= m; ++p)for (int q = p; q <= m; ++q)for (int o = 1; o <= t; ++o) f[i][j][p][q][o] = -1;memset(Sum, 0, sizeof(Sum));for (int i = 1; i <= n; ++i) {for (int j = 1; j <= m; ++j) {scanf("%d", &Num);Sum[i][j] = Sum[i][j - 1] + Sum[i - 1][j] - Sum[i - 1][j - 1] + Num;}}Ave = (DB)Sum[n][m] / (DB)t;Solve(1, n, 1, m, t);printf("%.2lf\n", sqrt(f[1][n][1][m][t] / t));return 0;
}