#include <iostream>
#include <string>
#include <math.h>
#include <algorithm>
#include <vector>
#include <string.h>
#include <numeric>
#include <iostream>
#include <queue>
#include <assert.h>
#include <map>
#include <set>
#include <limits.h>
#include <random>
#include <chrono>
using namespace std;
#define ll long long
#define ld long double
const int MX = 5005;
const int BLOCK = 105;
const ll inf = 8000000000000000069ll;
const ll mod = 1e9 + 7;
const ll inv2 = (mod + 1) / 2;
const int dxh[] = {1, 1, -1, -1, 2, 2, -2, -2};
const int dyh[] = {2, -2, 2, -2, 1, -1, 1, -1}; // horse
const int dx[] = {1, -1, 0, 0, 0, 0};
const int dy[] = {0, 0, 1, -1, 0, 0}; // adj
const int dz[] = {0, 0, 0, 0, 1, -1};
const int dxd[] = {1, 1, 1, 0, -1, -1, -1, 0};
const int dyd[] = {1, 0, -1, -1, -1, 0, 1, 1}; // diag
mt19937 rng(time(NULL));
int n, m, k;
string s[MX];
int wt[MX];
ll tot[MX][4];
int get(char c){
if(c == 'A') return 0;
if(c == 'C') return 1;
if(c == 'G') return 2;
return 3;
}
int main(){
cin.tie(0) -> sync_with_stdio(0);
cin >> n >> m >> k;
for(int i = 1; i <= n; i ++)
cin >> s[i];
for(int i = 1; i <= n; i ++)
wt[i] = rng();
for(int i = 1; i <= n; i ++)
for(int j = 0; j < m; j ++)
tot[j][get(s[i][j])] += wt[i];
for(int i = 1; i <= n; i ++){
ll c1 = 0ll, c2 = 0ll;
for(int j = 1; j <= n; j ++) if(i != j)
c1 += k * 1ll * wt[j];
for(int j = 0; j < m; j ++)
for(int v = 0; v < 4; v ++)
if(get(s[i][j]) != v)
c2 += tot[j][v];
if(c1 == c2){
cout << i << endl;
return 0;
}
}
cout << -1 << endl;
return 0;
}
