#include <bits/stdc++.h>
using namespace std;
const int M = 4105, N = 4105;
int m, n, x, sum[M][N][4];
char s[M][N];
vector<int> ve;
random_device rd;
mt19937_64 mt(rd());
int convert(char c) {
switch (c) {
case 'A': return 0;
case 'T': return 1;
case 'G': return 2;
case 'C': return 3;
default: return 0;
}
}
bool check(int ind, int pos) {
int cnt = (pos - (ind <= pos)) * x;
for (int i = 1; i <= n; i++) {
cnt -= sum[pos][i][convert(s[ind][i])];
}
return cnt == 0;
}
int main() {
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
cin >> m >> n >> x;
for (int i = 1; i <= m; i++) {
cin >> (s[i] + 1);
for (int j = 1; j <= n; j++) {
int cur = convert(s[i][j]);
for (int k = 0; k < 4; k++) {
sum[i][j][k] = sum[i - 1][j][k] + (cur != k);
}
}
ve.push_back(m + 1 - i);
}
shuffle(ve.begin() + 1, ve.end(), mt);
for (int i = 1; i <= m; i++) {
bool chk = true;
for (int &v : ve) {
if (!check(i, v)) {
chk = false;
break;
}
}
if (chk) {
return cout << i, 0;
}
}
}
