#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define fi first
#define se second
#define pb push_back
#define vi vector<int>
#define vl vector<ll>
#define pi pair<int, int>
#define pl pair<ll,ll>
#define all(x) (x).begin(),(x).end()
const int maxn=4110;
vector<bitset<4*maxn>> ss(maxn);
vector<string> s;
vector<vi> dif;
vi ok;
int n,m,k;
int diff(int i, int j) {
if (dif[i][j]!=-1) {
return dif[i][j];
}
int cnt=m-(ss[i]&ss[j]).count();
dif[i][j]=cnt;
dif[j][i]=cnt;
if (cnt!=k) {
ok[i]=0;
ok[j]=0;
}
return cnt;
}
int32_t main() {
ios::sync_with_stdio(0);
cin.tie(0);
cin >> n >> m >> k;
s.resize(n);
dif.resize(n,vi(n,-1));
ok.resize(n,1);
for (int i=0; i<n; i++) {
cin >> s[i];
for (int j=0; j<m; j++) {
if (s[i][j]=='A') {
ss[i].set(j);
}
else if (s[i][j]=='C') {
ss[i].set(j+m);
}
else if (s[i][j]=='G') {
ss[i].set(j+2*m);
}
else {
ss[i].set(j+3*m);
}
}
}
vector<map<char,int>> cnts(m);
for (int i=0; i<n; i++) {
for (int j=0; j<m; j++) {
cnts[j][s[i][j]]++;
}
}
mt19937 rnd(time(0));
vi ord1(n);
iota(all(ord1),0);
shuffle(all(ord1),rnd);
for (auto i:ord1) {
if (!ok[i]) {
continue;
}
int tot=0;
for (int j=0; j<m; j++) {
tot+=n-cnts[j][s[i][j]];
}
if (tot!=k*(n-1)) {
continue;
}
vi ord(n);
iota(all(ord),0);
shuffle(all(ord),rnd);
for (auto j:ord) {
if (i==j) {
continue;
}
if (diff(i,j)!=k) {
break;
}
}
if (ok[i]) {
cout << i+1 << '\n';
return 0;
}
}
return 0;
}
