// I am now here, but I have yet to prove that I am worthy of my place here.
#include <bits/stdc++.h>
using namespace std;
#pragma GCC optimize("Ofast")
#pragma GCC optimize("unroll-loops")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
using ll = long long;
const ll maxN = 4101;
const ll mod = 1'690'758'499;
const ll maxW = 420'000;
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
inline ll getRand(ll l, ll r) {
return uniform_int_distribution<ll>(l, r)(rng);
}
inline void maddto(ll &x, ll y) {x += y; if (x > mod) x -= mod;}
inline ll msub(ll x, ll y) {x -= y; if (x < 0) x += mod; return x;}
inline ll mmul(ll x, ll y) {x *= y; x %= mod; return x;}
ll N, M, K;
ll DNA[4][maxN][maxN];
ll corr[256];
char buf;
ll w[maxN], sumW;
ll columnSum[4][maxN];
int main() {
corr[(ll)'A'] = 0;
corr[(ll)'C'] = 1;
corr[(ll)'G'] = 2;
corr[(ll)'T'] = 3;
scanf("%lld %lld %lld", &N, &M, &K);
K = M - K;
for (ll i = 1; i <= N; i++) {
for (ll j = 1; j <= M; j++) {
scanf(" %c", &buf);
DNA[corr[(ll)buf]][i][j] = 1;
}
w[i] = getRand(1, maxW);
maddto(sumW, w[i]);
}
for (ll j = 1; j <= M; j++) {
for (ll d = 0; d < 4; d++) {
for (ll i = 1; i <= N; i++) {
maddto(columnSum[d][j], w[i] * DNA[d][i][j]);
}
}
}
for (ll comp = 1; comp <= N; comp++) {
ll target = mmul(K, msub(sumW, w[comp]));
ll total = 0;
for (ll d = 0; d < 4; d++) {
for (ll col = 1; col <= N; col++) {
maddto(total, mmul(DNA[d][comp][col], msub(columnSum[d][col], mmul(w[comp], DNA[d][comp][col]))));
}
}
if (target == total) {
printf("%lld\n", comp);
return 0;
}
}
}
