#include <bits/stdc++.h>
#define ll long long
#define pii pair<int, int>
#define pll pair<ll, ll>
#define F first
#define S second
using namespace std;
const ll MOD = 1e9 + 7;
ll N, M, K;
string s[4105];
ll arr[4105][4105], w[4105], cnt[4105][5];
mt19937 rd(chrono::steady_clock::now().time_since_epoch().count());
signed main()
{
ios::sync_with_stdio(0), cin.tie(0);
cin >> N >> M >> K;
for (int i = 1; i <= N; i++)
cin >> s[i];
for (int i = 1; i <= N; i++)
for (int j = 0; j < M; j++)
if (s[i][j] == 'A')
arr[i][j + 1] = 1;
else if (s[i][j] == 'T')
arr[i][j + 1] = 2;
else if (s[i][j] == 'C')
arr[i][j + 1] = 3;
else if (s[i][j] == 'G')
arr[i][j + 1] = 4;
for (int i = 1; i <= N; i++)
w[i] = rd() % MOD;
for (int i = 1; i <= N; i++)
for (int j = 1; j <= M; j++)
cnt[j][arr[i][j]] += w[i];
for (int i = 1; i <= N; i++)
{
ll totc = 0, totw = 0;
for (int j = 1; j <= N; j++)
if(i != j)
totw = (totw + w[j]) % MOD;
totw = totw * K % MOD;
for (int j = 1; j <= M; j++)
{
for (int k = 1; k <= 4; k++)
if(k != arr[i][j])
totc = (totc + cnt[j][k]) % MOD;
}
if(totc == totw)
{
cout << i << '\n';
return 0;
}
}
}
