#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
#define MASK(i) (1ULL << (i))
#define GETBIT(mask, i) (((mask) >> (i)) & 1)
#define ALL(v) (v).begin(), (v).end()
ll max(ll a, ll b){return (a > b) ? a : b;}
ll min(ll a, ll b){return (a < b) ? a : b;}
ll gcd(ll a, ll b){return __gcd(a, b);}
ll lcm(ll a, ll b){return a / gcd(a, b) * b;}
ll LASTBIT(ll mask){return (mask) & (-mask);}
int pop_cnt(ull mask){return __builtin_popcountll(mask);}
int ctz(ull mask){return __builtin_ctzll(mask);}
int logOf(ull mask){return 63 - __builtin_clzll(mask);}
mt19937_64 rng(chrono::high_resolution_clock::now().time_since_epoch().count());
ll rngesus(ll l, ll r){return l + (ull) rng() % (r - l + 1);}
double rngesus_d(double l, double r){
double cur = rngesus(0, MASK(60) - 1);
cur /= MASK(60) - 1;
return l + cur * (r - l);
}
template <class T1, class T2>
bool maximize(T1 &a, T2 b){
if (a < b) {a = b; return true;}
return false;
}
template <class T1, class T2>
bool minimize(T1 &a, T2 b){
if (a > b) {a = b; return true;}
return false;
}
template <class T>
void printArr(T container, string separator = " ", string finish = "\n", ostream &out = cout){
for(auto item: container) out << item << separator;
out << finish;
}
template <class T>
void remove_dup(vector<T> &a){
sort(ALL(a));
a.resize(unique(ALL(a)) - a.begin());
}
#pragma GCC target("popcnt,lzcnt")
const int N = 4100;
vector<int> get_possible(int n, int m, int k, int idx, vector<bitset<N * 4>> &s){
vector<int> ans;
for(int i = 0; i < n; ++i) if (i != idx){
int cnt = m - (s[i] & s[idx]).count();
if (cnt == k) ans.push_back(i);
}
return ans;
}
char convert(char c){
switch (c){
case 'A':
return '0';
case 'C':
return '1';
case 'G':
return '2';
case 'T':
return '3';
}
return '4';
}
int solve(){
int n, m, k; cin >> n >> m >> k;
vector<pair<string, int>> s(n);
for(int i = 0; i < n; ++i) {
cin >> s[i].first;
for(char &c: s[i].first) c = convert(c);
s[i].second = i + 1;
}
shuffle(ALL(s), rng);
vector<int> cur;
int sum[m][8]; memset(sum, 0, sizeof sum);
for(int i = 0; i< n; ++i) for(int j = 0; j< m; ++j){
int digit = s[i].first[j] - '0';
sum[j][digit]++;
if (i < n / 2) sum[j][digit + 4]++;
}
for(int i = 0; i < n; ++i){
int cnt = n * m, _cnt = (n / 2) * m;
for(int j = 0; j < m; ++j){
int digit = s[i].first[j] - '0';
cnt -= sum[j][digit];
_cnt -= sum[j][digit + 4];
}
int target = (n / 2) * k;
if (i < n / 2) target -= k;
if (cnt == (n-1) * k && _cnt == target) cur.push_back(i);
}
vector<bitset<N * 4>> giga_nigga(n);
for(int i = 0; i < n; ++i){
for(int j = 0; j < m; ++j){
int digit = s[i].first[j] - '0';
giga_nigga[i][j + digit * N] = 1;
}
}
while(cur.size()){
int u = cur.back(); cur.pop_back();
vector<int> wtf = get_possible(n, m, k, u, giga_nigga);
if (wtf.size() == s.size() - 1) return s[u].second;
vector<int> tmp;
for(int i: wtf) if (binary_search(ALL(cur), i)) tmp.push_back(i);
cur = tmp;
}
return -1;
}
int main(void){
ios::sync_with_stdio(0);cin.tie(0); cout.tie(0);
clock_t start = clock();
cout << solve() << "\n";
cerr << "Time elapsed: " << clock() - start << " ms\n";
return 0;
}
