#include <iostream>
#include <random>
using namespace std;
typedef unsigned int ui;
const int N = 4100;
int g(char c) { return c == 'A' ? 0 : c == 'C' ? 1 : c == 'G' ? 2 : 3; }
mt19937 rng(1234);
int main() {
ios::sync_with_stdio(0), cin.tie(0);
int n, m, k;
cin >> n >> m >> k;
static string s[N];
static ui h[N], f[N][4];
for (int i = 0; i < n; i++) {
cin >> s[i];
h[i] = rng();
for (int j = 0; j < m; j++) f[j][g(s[i][j])] += h[i];
}
ui sum = 0;
for (int i = 0; i < n; i++) sum += h[i];
for (int i = 0; i < n; i++) {
ui x = 0;
for (int j = 0; j < m; j++) x += f[j][g(s[i][j])];
if ((m - k) * (sum - h[i]) == x - m * h[i]) {
cout << i + 1 << '\n';
return 0;
}
}
cout << -1 << '\n';
return 0;
}
