#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;
}
}
// Turn input into binary string
M = 4 * M;
K = 2 * K;
// bit 0 = -1
// bit 1 = +1
// Take \sum A[x][i] * A[y][i]
// (M - K) - K = differ
// differ = M - 2K
K = M - 2 * K;
mt19937 rnd(123456789);
vector<int> weight(N);
for (int i = 0; i < N; i++) {
weight[i] = (rnd()) % (1ll << 30);
}
long long sum_weight = 0;
vector<long long> sum(M);
for (int i = 0; i < N; i++) {
sum_weight += weight[i];
for (int j = 0; j < M; j++) {
sum[j] += (A[i][j] == 0 ? -1 : 1) * weight[i];
}
}
for (int i = 0; i < N; i++) {
long long hashed = 0;
for (int j = 0; j < M; j++) {
int sgn = (A[i][j] == 0 ? -1 : 1);
hashed += 1ll * sgn * (sum[j] - sgn * weight[i]);
}
long long should_be = 1ll * (sum_weight - weight[i]) * K;
if (hashed == should_be) {
cout << i + 1 << '\n';
return 0;
}
}
cout << -1 << '\n';
return 0;
}
