#include <bits/stdc++.h>
using namespace std;
int main() {
ios::sync_with_stdio(0);
cin.tie(0);
int N, M, K;
cin >> N >> M >> K;
const int MAX = 16405;
vector<bitset<MAX>> A(N, 0);
for (int i = 0; i < N; i++) {
string s;
cin >> s;
for (int j = 0; j < M; j++) {
if (s[j] == 'A') A[i][j * 4 + 0] = 1;
if (s[j] == 'C') A[i][j * 4 + 1] = 1;
if (s[j] == 'T') A[i][j * 4 + 2] = 1;
if (s[j] == 'G') A[i][j * 4 + 3] = 1;
}
}
bitset<MAX> candidates = 0;
for (int i = 0; i < N; i++) {
candidates[i] = 1;
}
mt19937 rnd(MAX);
vector<int> ord(N);
iota(begin(ord), end(ord), 0);
const int L = 40;
while (candidates.count() > L) {
shuffle(begin(ord), end(ord), rnd);
for (int i = 0; i + 1 < N; i++) {
int x = ord[i + 0];
int y = ord[i + 1];
int differ = (A[x] ^ A[y]).count() / 2;
if (differ != K) {
candidates[x] = 0;
candidates[y] = 0;
}
}
}
vector<int> res;
for (int i = 0; i < N; i++) {
if (candidates[i]) {
res.emplace_back(i);
}
}
for (int x : res) {
bool good = true;
for (auto y : ord) if (x != y) {
int differ = (A[x] ^ A[y]).count() / 2;
if (differ != K) {
good = false;
break;
}
}
if (good) {
cout << (x + 1) << '\n';
return 0;
}
}
cout << -1 << '\n';
return 0;
}
