// source problem :
#include <bits/stdc++.h>
using namespace std;
#define all(x) x.begin(), x.end()
#define int unsigned long long
#define lb lower_bound
#define ub upper_bound
#define MASK(i) (1LL << (i))
const int inf = 1e18;
void ckmax(int& f, int s)
{
f = (f > s ? f : s);
}
void ckmin(int& f, int s)
{
f = (f < s ? f : s);
}
auto seed = chrono::high_resolution_clock::now().time_since_epoch().count();
mt19937_64 mt(seed);
int32_t main()
{
ios_base::sync_with_stdio(false);
cin.tie(0);
int n, m, k;
cin >> n >> m >> k;
vector<string> a(n);
string DNA = "ACGT";
for (string &s : a)
{
cin >> s;
for (char &c : s)
{
c = (char)('0' + DNA.find(c));
}
}
vector<int> hash_val(n);
int tot_row = 0, row = 0;
for (int &i : hash_val)
{
i = mt();
row += i;
tot_row += i * m;
}
vector<vector<int>> hash_col(m, vector<int>(4));
for (int i = 0; i < n; i++)
{
for (int j = 0; j < m; j++)
{
hash_col[j][a[i][j] - '0'] += hash_val[i];
}
}
for (int i = 0; i < n; i++)
{
int tot = tot_row;
for (int j = 0; j < m; j++)
{
tot -= hash_col[j][a[i][j] - '0'];
}
if (tot == (row - hash_val[i]) * k)
{
cout << i + 1;
return 0;
}
}
return 0;
}
