#include <bits/stdc++.h>
using namespace std;
#define fast ios::sync_with_stdio(0);cin.tie(0);
#define s second
#define f first
typedef long long ll;
const ll MOD = 1e9+9;
const ll LOGN = 20;
const ll MAXN = 4100 + 100;
const ll P = 31;
ll cnt[MAXN][4];
ll need = 0;
string dna[MAXN];
int main() {
fast
int N, M, K;
cin >> N >> M >> K;
ll now = 1;
for (int i = 1; i <= N; i++) {
now = (now * P) % MOD;
need = (need + (now * K % MOD)) % MOD;
cin >> dna[i];
dna[i] = "#" + dna[i];
for (int j = 1; j <= M; j++) {
if (dna[i][j] == 'A')
cnt[j][0] = (cnt[j][0] + now) % MOD;
else if (dna[i][j] == 'T')
cnt[j][1] = (cnt[j][1] + now) % MOD;
else if (dna[i][j] == 'G')
cnt[j][2] = (cnt[j][2] + now) % MOD;
else if (dna[i][j] == 'C')
cnt[j][3] = (cnt[j][3] + now) % MOD;
}
}
now = 1;
for (int i = 1; i <= N; i++) {
now = (now * P) % MOD;
ll need_now = need - (now * K % MOD);
while (need_now < 0)
need_now += MOD;
ll calc = 0;
for (int j = 1; j <= M; j++) {
if (dna[i][j] == 'A')
calc = (calc + (cnt[j][1] + cnt[j][2] + cnt[j][3])) % MOD;
else if (dna[i][j] == 'T')
calc = (calc + (cnt[j][0] + cnt[j][2] + cnt[j][3])) % MOD;
else if (dna[i][j] == 'G')
calc = (calc + (cnt[j][0] + cnt[j][1] + cnt[j][3])) % MOD;
else if (dna[i][j] == 'C')
calc = (calc + (cnt[j][0] + cnt[j][1] + cnt[j][2])) % MOD;
}
if (calc == need_now) {
cout << i << "\n";
return 0;
}
}
}
