#include <bits/stdc++.h>
using namespace std;
using i64 = long long;
using ll = long long;
#define pb emplace_back
#define X first
#define Y second
#define AI(i) begin(i), end(i)
template<class T> bool chmax(T &a, T b) { return a < b && (a = b, true);}
template<class T> bool chmin(T &a, T b) { return b < a && (a = b, true);}
#ifdef KEV
#define DE(args...) kout("[ " + string(#args) + " ] = ", args)
void kout() { cerr << endl; }
template<class T, class ...U> void kout(T a, U ...b) { cerr << a << ' ', kout(b...); }
template<class T> void debug(T l, T r) { while (l != r) cerr << *l++ << " \n"[l==r]; }
#else
#define DE(...) 0
#define debug(...) 0
#endif

const int MAX_N = 300010;
int main() {
	ios_base::sync_with_stdio(0), cin.tie(0);

	mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());

	auto solve = [&]() {
		int n, m, k;
		cin >> n >> m >> k;

		vector<string> a(n);
		for (int i = 0; i < n; i++) {
			cin >> a[i];
		}

		vector<int> ok(n, true);

		for (int rep = 0; count(ok.begin(), ok.end(), true) > 1; rep++) {
			vector<int> inds;
			for (int i = 0; i < n; i++) {
				if (rng() % 2 == 0) {
					inds.push_back(i);
				}
			}

			int N = inds.size();
			vector<array<int, 4>> cnt(m);
			for (auto i : inds) {
				for (int j = 0; j < m; j++) {
					cnt[j][a[i][j] - 'A']++;
				}
			}

			for (auto i : inds) {
				i64 tot = 0;
				for (int j = 0; j < m; j++) {
					tot += N - cnt[j][a[i][j] - 'A'];
				}
				if (tot != 1LL * (N - 1) * k) {
					ok[i] = false;
				}
			}
		}

		assert(count(ok.begin(), ok.end(), true) == 1);

		int ans = find(ok.begin(), ok.end(), true) - ok.begin();
		cout << ans + 1 << '\n';
	};

	solve();
}