///In the name of GOD
//#pragma GCC optimize("O2")
#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
const ll MXN = 4100 + 10;
const ll Mab = 4783;
const ll Mod = 1e9 + 87;
ll n, m, k;
ll hsh[MXN][4], pw[MXN], one[MXN];
string S[MXN];
void mkay(ll& x){
if(x >= Mod) x -= Mod;
if(x < 0) x += Mod;
}
ll MAP(char c){
switch(c){
case 'A' : return 0;
case 'T' : return 1;
case 'C' : return 2;
case 'G' : return 3;
}
assert(0);
}
int main(){
ios::sync_with_stdio(0);cin.tie(0); cout.tie(0);
pw[0] = one[0] = 1;
for(int i = 1; i < MXN; i ++){
pw[i] = pw[i - 1] * Mab % Mod;
one[i] = (one[i - 1] + pw[i]) % Mod;
}
cin >> n >> m >> k;
for(int i = 0; i < n; i ++) cin >> S[i];
for(int i = 0; i < n; i ++){
for(int j = 0; j < m; j ++){
ll ch = MAP(S[i][j]);
hsh[j][ch] += pw[i];
mkay(hsh[j][ch]);
}
}
for(int j = 0; j < m; j ++) for(int c = 0; c < 4; c ++){
hsh[j][c] = one[n - 1] - hsh[j][c];
mkay(hsh[j][c]);
}
ll exp = one[n - 1] * k % Mod;
for(int i = 0; i < n; i ++){
ll diff = 0;
for(int j = 0; j < m; j ++){
ll ch = MAP(S[i][j]);
diff += hsh[j][ch];
mkay(diff);
}
exp -= pw[i] * k % Mod, mkay(exp);
if(diff == exp) return cout << i + 1 << '\n', 0;
exp += pw[i] * k % Mod, mkay(exp);
}
cout << "wtmoo" << '\n';
return 0;
}
/*!
HE'S AN INSTIGATOR,
ENEMY ELIMINATOR,
AND WHEN HE KNOCKS YOU BETTER
YOU BETTER LET HIM IN.
*/
//! N.N
