#include <bits/stdc++.h>
#define pb push_back
#define f first
#define sc second
using namespace std;
typedef long long int ll;
typedef string str;
unordered_map<char, int> dna = {{'A', 0}, {'T', 1}, {'C', 2}, {'G', 3}};
int n, m, k;
bitset<4100> candidate;
char s[4100][4100];
int cnt[4100][4];
int comp(int a, int b){
int c = 0;
for(int i = 0; i < m; i++) if(s[a][i] != s[b][i]) c++;
return c;
}
int ans = -1;
void dfs(int nd){
if(ans != -1) return;
if(!candidate[nd]) return;
int nxt = -1;
bool ok = 1;
for(int i = 0; i < n; i++) if(i != nd && candidate[i]){
int c = comp(i, nd);
if(c == k){
nxt = i;
if(!ok){
dfs(i);
break;
}
}
else{
ok = 0;
candidate[nd] = 0;
candidate[i] = 0;
if(nxt != -1){
dfs(nxt);
break;
}
}
}
if(!ok) return;
for(int i = 0; i < n; i++) if(i != nd && !candidate[i]){
int c = comp(i, nd);
if(c != k){
ok = 0;
break;
}
}
if(ok) ans = nd;
else candidate[nd] = 0, dfs(nxt);
}
int main(){
ios_base::sync_with_stdio(0);
cin.tie(0);
cin >> n >> m >> k;
candidate.flip();
for(int i = 0; i < n; i++) for(int j = 0; j < m; j++) cin >> s[i][j];
for(int i = 0; i < n; i++) for(int j = 0; j < m; j++){
if(!i) cnt[j][0] = 0, cnt[j][1] = 0, cnt[j][2] = 0, cnt[j][3] = 0;
cnt[j][dna[s[i][j]]]++;
}
for(int i = 0; i < n; i++){
int c = 0;
for(int j = 0; j < m; j++) for(int x = 0; x < 4; x++) if(x != dna[s[i][j]]) c+=cnt[j][x];
if(c != (n-1)*k) candidate[i] = 0;
}
for(int i = 0; i < n; i++) if(candidate[i]){
dfs(i);
break;
}
cout << ans+1 << "\n";
}
