#include <iostream>
#include <cstring>
#include <algorithm>
#include <vector>
#include <map>
#define x first
#define y second
using namespace std;
typedef long long LL;
typedef unsigned long long ULL;
typedef pair<LL, int> PII;
const int N = 1e5 + 10, P = 13331;
int n, m, k;
string str[N];
ULL f[4][N], p[N];
int main()
{
    ios::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
    cin >> n >> m >> k;
    p[0] = 1;
    for (int i = 1; i <= n; i++)
        p[i] = p[i - 1] * P;
    for (int i = 1; i <= n; i++)
    {
        cin >> str[i];
        for (int j = 0; j < m; j++)
        {
            int t = str[i][j] - 'A';
            for (int k = 0; k < 4; k++)
                f[k][j] = f[k][j] * P + (k == t);
        }
    }
    ULL tot = 0, aim = 0;
    for (int j = 0; j < m; j++)
        for (int k = 0; k < 4; k++)
            tot += f[k][j];
    for (int i = 1; i <= n; i++)
        aim = aim * P + k;
    // for (int j = 0; j < m; j++)
    // {
    //     for (int k = 0; k < 4; k++)
    //         cout << f[k][j] << ',';
    //     cout << endl;
    // }
    // cout << tot << ' ' << aim << endl;
    for (int i = 1; i <= n; i++)
    {
        ULL cur = tot;
        for (int j = 0; j < m; j++)
            cur -= f[str[i][j] - 'A'][j];
        // cout << ':' << cur << ',' << aim - k * p[n - i] << endl;
        if (cur == aim - k * p[n - i])
        {
            cout << i << endl;
            return 0;
        }
    }
    return 0;
}