#include <bits/stdc++.h> 
using i64 = long long;
constexpr int N = 1e5 + 5;
constexpr i64 P = 19260817;
int n, m, k;
i64 w[N], f[N][4];
std::mt19937_64 rnd(std::chrono::steady_clock::now().time_since_epoch().count());
std::uniform_int_distribution<i64> dis(1, P);
std::string s[N];
/*
5 10 2
DCDDDCCADA
ACADDCCADA
DBADDCCBDC
DBADDCCADA
ABADDCCADC
*/
int main() {
	std::ios::sync_with_stdio(false);
	std::cin.tie(nullptr);
	std::cin >> n >> m >> k;
	i64 sum = 0;
	for (int i = 1; i <= n; ++i) {
		w[i] = dis(rnd);
//		std::cout << w[i] << '\n';
		sum = sum + w[i];
		std::cin >> s[i];
		for (int j = 0; j < m; ++j) f[j][s[i][j] - 'A'] += w[i];
	}
	for (int i = 1; i <= n; ++i) {
		i64 nw = 0;
		for (int j = 0; j < m; ++j)	 nw += f[j][s[i][j] - 'A'];
		if (nw == 1ll * (m - k) * sum + 1ll * k * w[i]) std::cout << i << '\n', exit(0);
	}	
	std::cout << "-1\n";
	return 0;
}