#include <bits/stdc++.h>
using i64 = long long;
constexpr i64 P = (1ll << 61) - 1;
constexpr std::array<char, 4> cst = {'A', 'C', 'G', 'T'};
void add(i64 &a, i64 b) {
a += b;
if (a >= P) {
a -= P;
}
}
int main() {
std::ios::sync_with_stdio(false);
std::cin.tie(nullptr);
int N, M, K;
std::cin >> N >> M >> K;
std::vector<std::string> S(N);
for (auto &e : S) {
std::cin >> e;
}
std::vector<i64> H(N);
std::mt19937 mt(100);
std::uniform_int_distribution<i64> dist(0, P - 1);
for (auto &e : H) {
e = dist(mt);
}
std::vector<std::array<i64, 4>> hl(N);
auto get_id = [&](char x) {
if (x == 'A') return 0;
if (x == 'C') return 1;
if (x == 'G') return 2;
return 3;
};
for (int i = 0; i < N; ++i) {
for (int j = 0; j < M; ++j) {
add(hl[j][get_id(S[i][j])], H[i]);
}
}
for (int i = 0; i < N; ++i) {
i64 sum = 0;
for (int j = 0; j < M; ++j) {
const int v = get_id(S[i][j]);
for (int x = 0; x < 4; ++x) {
if (x == v) {
continue;
}
add(sum, hl[j][x]);
}
}
i64 exp = 0;
for (int j = 0; j < N; ++j) {
if (i == j) {
continue;
}
add(exp, H[j]);
}
exp = (i64)(__int128_t(exp) * __int128_t(K) % __int128_t(P));
if (exp == sum) {
std::cout << i + 1 << std::endl;
break;
}
}
}
