#include <bits/stdc++.h>
typedef long long ll;
#define f first
#define s second
#define pb push_back
#define ep emplace
#define eb emplace_back
#define lb lower_bound
#define ub upper_bound
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()
#define uniquev(v) sort(all(v)), (v).resize(unique(all(v)) - (v).begin())
#define mem(f,x) memset(f , x , sizeof(f))
#define sz(x) (int)(x).size()
#define __lcm(a, b) (1ll * ((a) / __gcd((a), (b))) * (b))
#define mxx *max_element
#define mnn *min_element
#define cntbit(x) __builtin_popcountll(x)
using namespace std;
const int N = 4200;
mt19937 rd(chrono::steady_clock::now().time_since_epoch().count());
ll get(ll l, ll h) {
return uniform_int_distribution <ll> (l, h)(rd);
}
string s[N];
int get(char ch) {
if (ch == 'A')
return 0;
if (ch == 'C')
return 1;
if (ch == 'G')
return 2;
return 3;
}
ll all[N][4], val[N];
int solve() {
ll k;
int n, m;
cin >> n >> m >> k;
ll sum = 0;
for (int i = 1; i <= n; i++) {
cin >> s[i];
val[i] = get(1, 1000000000);
sum += val[i] * k;
for (int j = 0; j < m; j++) {
all[j][get(s[i][j])] += val[i];
}
}
for (int i = 1; i <= n; i++) {
ll diff = 0;
for (int j = 0; j < m; j++) {
int cur = get(s[i][j]);
for (int k = 0; k < 4; k++) {
if (k == cur)
continue;
diff += all[j][k];
}
}
if (diff == (sum - val[i] * k)) {
return i;
}
}
return -1;
}
int main() {
ios_base::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
int t = 1;
//cin >> t;
while (t--) {
cout << solve() << '\n';
}
}
/*
4 3
2 3
2 6 2
2 4 2
6 5 1
1 2 3
*/
