#ifdef _DEBUG
#define _GLIBCXX_DEBUG
#endif
#define _CRT_SECURE_NO_WARNINGS
#include <bits/stdc++.h>

using namespace std;

typedef long long ll;
typedef pair<int, int> pii;
typedef vector<int> vi;
typedef vector<ll> vll;
typedef long double ld;

#define mk make_pair
#define inb push_back
#define X first
#define Y second
#define all(v) v.begin(), v.end()
#define sqr(x) ((x) * (x))
#define TIME 1.0 * clock() / CLOCKS_PER_SEC
#define y1 AYDARBOG
//continue break pop_back return scanf

int solve();

int main()
{
    //ios_base::sync_with_stdio(0);
    //cin.tie(0);
#define TASK "chevaux"
#ifndef _DEBUG
    //freopen(TASK".in", "r", stdin), freopen(TASK".out", "w", stdout);
#endif
    solve();
#ifdef _DEBUG
    fprintf(stderr, "\nTIME: %.3f\n", TIME);
#endif
}

const int BUFSZ = (int)4e5 + 7;
char buf[BUFSZ];
string get_str()
{
    scanf(" %s", buf);
    return string(buf);
}

const int INF = 1e9 + 7;

int solve()
{
    int n, m, k;
    scanf("%d %d %d", &n, &m, &k);
    char a[n][m + 1];
    for (int i = 0; i < n; ++i)
    {
        scanf(" %s", a[i]);
    }
    vector<pii> swaps(m / 2, mk(0, 2));
    int ans = 0;
    pii solo = mk(0, 0);
    if (m & 1)
    {
        solo = mk(0, 1);
        for (int i = 0; i < n / 2; ++i)
        {
            if (a[i][m / 2] != a[n - i - 1][m / 2])
            {
                ++solo.X;
            }
        }
    }
    for (int i = 0; i < n; ++i)
    {
        int ri = n - i - 1;
        for (int j = 0; j < m / 2; ++j)
        {
            int rj = m - j - 1;
            if (a[i][j] != a[i][rj])
            {
                ++ans;
                //fprintf(stderr, "%d %d :: %c %c\n", i, j, a[i][j], a[i][rj]);
            }
            if (i < n / 2)
            {
                char c1 = a[i][j];
                char c2 = a[i][rj];
                char c3 = a[ri][j];
                char c4 = a[ri][rj];
                //fprintf(stderr, "%c %c\n%c %c\n", c1, c2, c3, c4);
                //c1 c2
                //c3 c4
                //c1 == c2 c3 == c4
                if (c1 == c2 && c3 == c4)
                {
                    if (c1 == c3)
                        swaps[j].X += 0;
                    else
                        swaps[j].X += 2;
                }
                if (c1 == c2 && c3 != c4)
                {
                    if (c3 != c1 && c3 != c2 && c4 != c1 && c4 != c2)
                        swaps[j].X += 1;
                }
                if (c1 != c2 && c3 == c4)
                {
                    if (c1 != c3 && c1 != c4 && c2 != c3 && c2 != c4)
                        swaps[j].X += 1;
                }
                if (c1 != c2 && c3 != c4)
                {
                    if (c1 != c3 && c1 != c4 && c2 != c3 && c2 != c4)
                        swaps[j].X += 1;
                }
                //fprintf(stderr, "%d\n", swaps[j].X);
            }
        }
    }
    /*for (auto cur : swaps)
    {
        printf("%d %d\n", cur.X, cur.Y);
    }//*/
    //printf("ANS : %d %d\n", ans, solo.X);
    sort(all(swaps), [&](pii x, pii y)
    {
        return x.X < y.X;
    });
    int cr = 0;
    for (auto x : swaps)
    {
        if (cr >= k)
            break;
        if (solo.Y)
        {
            if (solo.Y + cr >= k && cr + x.Y >= k)
            {
                ans += min(x.X, solo.X);
                printf("%d\n", ans);
                exit(0);
            }
        }
        cr += x.Y;
        ans += x.X;
        if (cr >= k)
            break;
    }
    if (solo.Y)
    {
        if (solo.Y + cr == k)
        {
            ans += solo.X;
            printf("%d\n", ans);
            exit(0);
        }
    }
    printf("%d\n", ans);
    return 0;
}
/*
4 5 5
cdcde
eeace
ddcda
bccbc
*/