#include<bits/stdc++.h>
using namespace std;
const long long inf = (long long) 1e18 + 10;
const int inf1 = (int) 1e9 + 10;
// #define int long long
#define dbl long double
#define endl '\n'
#define sc second
#define fr first
#define mp make_pair
#define pb push_back
#define all(x) x.begin(), x.end()
const int maxn = 4140;
mt19937 rng(143243);
int ran(int a, int b) {
return a + rng()%(b-a+1);
}
int n, m, k, a[maxn][maxn], cntcol[maxn][5], cntper[maxn][5];
string s[maxn];
void solve() {
cin >> n >> m >> k;
for(int i = 1; i <= n; i++) {
cin >> s[i];
for(int j = 0; j < m; j++) {
if(s[i][j] == 'A') a[i][j] = 0;
if(s[i][j] == 'C') a[i][j] = 1;
if(s[i][j] == 'G') a[i][j] = 2;
if(s[i][j] == 'T') a[i][j] = 3;
cntper[i][a[i][j]]++;
cntcol[j][a[i][j]]++;
}
}
for(int i = 1; i <= n; i++) {
int cntwrg = 0;
for(int j = 0; j < m; j++) {
cntwrg+= n-cntcol[j][a[i][j]];
}
bool ok = true;
if(cntwrg != k*(n-1)) ok = false;
// for(int j = 1; j <= n; j++) {
// if(i == j) continue;
// if(abs(cntper[i][0]-cntper[j][0])+abs(cntper[i][1]-cntper[j][1]) != 2*k) ok = false;
// cout << i << " " << j << " " << abs(cntper[i][0]-cntper[j][0])+abs(cntper[i][1]-cntper[j][1]) << endl;
// }
int rndtest = 500000;
while(rndtest-- && ok) {
int j = ran(1,n-1);
if(j >= i) j++;
int cnt = 0;
for(int x = 0; x < m; x++) {
if(a[i][x] != a[j][x]) cnt++;
}
if(cnt != k) {
ok = false;
}
}
if(ok) {
cout << i << endl;
return;
}
}
}
int32_t main() {
ios::sync_with_stdio(false); cin.tie(0);
// freopen("in.in", "r", stdin);
// freopen("out.out", "w", stdout);
int tt = 1;
// cin >> tt;
while(tt--) {
solve();
}
}
