#include <iostream>
#include <cstdio>
#include <cstdlib>
#include <iomanip>
#include <cmath>
#include <queue>
#include <stack>
#include <set>
#include <vector>
#include <cstring>
#include <algorithm>
#include <map>
#include <stack>
#include <random>
using namespace std;
typedef long long ll;
typedef __int128 ull;
const int N = 100005;
const ull mod = 998244192608171013ll;
string s[N];
ull val[N];
ull cnt[N][26];
mt19937_64 rd(time(0));
int main() {
    ios::sync_with_stdio(0);
    cin.tie(0);
    int n, m, k;
    cin >> n >> m >> k;
    ull sum = 0;
    for (int i = 1; i <= n; i++) {
        cin >> s[i];
        val[i] = rd();
        sum += val[i] * (m - k);
        sum %= mod;
        for (int j = 0; j < m; j++) {
            cnt[j][s[i][j] - 'A'] += val[i];
            cnt[j][s[i][j] - 'A'] %= mod;
        }
    }
    for (int i = 1; i <= n; i++) {
        ull now = 0;
        for (int j = 0; j < m; j++) {
            now += cnt[j][s[i][j] - 'A'] - val[i];
            now = (now % mod + mod) % mod;
        }
        if (now == ((sum - val[i] * (m - k)) % mod + mod) % mod) {
            printf("%d\n", i);
            return 0;
        }
        // printf("%d %lld %lld %lld\n", i, (ll)val[i], (ll)now, (ll)sum);
    }
    printf("-1\n");
    return 0;
}