#include <bits/stdc++.h>
using namespace std;

int main()
{
	int n, m, k;
	cin >> n >> m >> k;
	for (int i = 1; i <= n; i++)
	{
	    for (int j = 1; j <= m; j++)
	    {
	        char ch;
	        cin >> ch;
	        b[i][j] = b[i-1][j] + b[i][j-1] - b[i-1][j-1] + ch;
	    }
	}
	int minn = 1e9;
	for (int i = 1; i <= n; i++)
	{
	    for (int j = 1; j <= m; j++)
	    {
	        for (int k = i; k <= n; k++)
	        {
	            for (int l = j; l <= n; l++)
	            {
	                if (b[k][l] - b[i-1][l] - b[k][j-1] + b[i-1][j-1] >= k)
	                {
	                    minn = min(minn, b[k][l] - b[i-1][l] - b[k][j-1] + b[i-1][j-1]);
	                }
	            }
	        }
	    }
	}
	cout << minn;
	return 0;
}