#include <bits/stdc++.h>
using namespace std;
using ll = long long;
int n , m , k , chek[4101][26] , ans , tim , one , two; // abar koro ?
string a[4101];
vector<int> perm;
int diff( int me , int her ) {
int dif = 0;
for( int z = 0; z < m; z++ ) {
if( a[me][z] != a[her][z] ) dif++;
}
return dif;
}
void getans( int me ) {
int hey = 0;
for( int z = 0; z < n; z++ ) {
if( perm[z] != me ) {
hey = diff( me , perm[z] );
if( hey != k ) {
z = n;
}
}
if( z == n - 1 ) ans = me;
}
}
int main(){
ios::sync_with_stdio(false);
cin.tie(nullptr);
cin >> n >> m >> k;
ans = -1;
for( int z = 0; z < n; z++ ) perm.push_back(z);
random_device rd;
mt19937 g(rd());
shuffle(perm.begin(), perm.end(), g);
for( int z = 0; z < n; z++ ) {
cin >> a[z];
// cout << k << "\n";
for( int y = 0; y < m; y++ ) {
chek[y][(int)(a[z][y] - 'A')]++;
// if( k == 2 ) cout << "here" << z << y << "\n";
}
}
for( int z = 0; z < n; z++ ) {
one = 0;
for( int y = 0; y < m; y++ ) {
one += (n - chek[y][(int)(a[z][y] - 'A')] );
}
// cout << one << " " << (k * (n - 1)) << " " << n << " " << k << "\n";
if( one == (k * (n - 1)) ) {
getans( z );
// cout << z << "\n";
if( ans != -1 ) z = n;
}
}
cout << ans + 1 << "\n";
return 0;
}
