#include <bits/stdc++.h>
using namespace std;
constexpr int MAXN = 4105;
int dtoi(char c) {
if (c == 'A')
return 0;
if (c == 'G')
return 1;
if (c == 'C')
return 2;
if (c == 'T')
return 3;
exit(-1);
}
char itod(int x) {
return "AGCT"[x];
}
int main() {
cin.tie(nullptr)->sync_with_stdio(false);
int n, m, k;
cin >> n >> m >> k;
vector<string> C(n);
vector<bitset<MAXN>> b0(n), b1(n);
vector<vector<int>> cnt(m, vector<int>(4));
for (string &s : C) {
cin >> s;
for (int i = 0; i < m; i++) {
cnt[i][dtoi(s[i])]++;
}
}
for (int i = 0; i < n; i++) {
for (int j = 0; j < m; j++) {
b0[i][j] = dtoi(C[i][j]) & 1;
b1[i][j] = dtoi(C[i][j]) >> 1 & 1;
}
}
random_device rd;
mt19937_64 g(rd());
vector<int> p(n);
iota(p.begin(), p.end(), 0);
shuffle(p.begin(), p.end(), g);
auto ok = [&](int i) {
for (int j : p) {
if (j != i && (b0[i] ^ b0[j] | b1[i] ^ b1[j]).count() != k)
return false;
}
return true;
};
for (int i : p) {
int s = 0;
for (int j = 0; s <= (n - 1) * k && j < m; j++) {
for (int l = 0; l < 4; l++) {
if (l != dtoi(C[i][j]))
s += cnt[j][l];
}
}
if (s != (n - 1) * k)
continue;
if (ok(i)) {
cout << i + 1;
return 0;
}
}
cout << -1;
}
