/*
* @Author: hungeazy
* @Date:   2024-11-28 14:38:05
* @Last Modified by:   hungeazy
* @Last Modified time: 2024-11-28 14:58:58
*/
#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp> 
#include <ext/pb_ds/tree_policy.hpp> 
// #pragma GCC optimize("O3")  
// #pragma GCC optimize("unroll-loops")  
// #pragma GCC target("avx2,bmi,bmi2,popcnt,lzcnt")  
using namespace std;
using namespace __gnu_pbds; 
#define fast ios_base::sync_with_stdio(false); cin.tie(NULL); cout.tie(NULL);
// #define int long long
#define ll long long 
#define ull unsigned long long
#define sz(x) x.size()
#define sqr(x) (1LL * (x) * (x))
#define all(x) x.begin(), x.end()
#define fill(f,x) memset(f,x,sizeof(f))
#define FOR(i,l,r) for(int i=l;i<=r;i++)
#define FOD(i,r,l) for(int i=r;i>=l;i--)
#define debug(x) cout << #x << " = " << x << '\n'
#define ii pair<int,int>
#define iii pair<int,ii>
#define di pair<ii,ii>
#define vi vector<int>
#define vii vector<ii>
#define mii map<int,int>
#define fi first
#define se second
#define pb push_back
#define MOD 1000000007
#define __lcm(a,b) (1ll * ((a) / __gcd((a), (b))) * (b))
#define YES cout << "YES\n"
#define NO cout << "NO\n"
#define MASK(i) (1LL << (i))
#define c_bit(i) __builtin_popcountll(i)
#define BIT(x,i) ((x) & MASK(i))
#define SET_ON(x,i) ((x) | MASK(i))
#define SET_OFF(x,i) ((x) & ~MASK(i))
#define oo 1e18
#define name ""
#define endl '\n'
// #define time() cerr << endl << "-------------Time:" << 1000.0 * clock() / CLOCKS_PER_SEC << "ms.";
template<typename T> bool maximize(T &res, const T &val) { if (res < val){ res = val; return true; }; return false; }
template<typename T> bool minimize(T &res, const T &val) { if (res > val){ res = val; return true; }; return false; }
template <class T> using ordered_set = tree <T, null_type, less_equal <T>, rb_tree_tag,tree_order_statistics_node_update>;
mt19937_64 rnd;
const int N = (int)1e5+10;
int n,m,k;
ull num[N][4],val[N];
string s[N];

int randRange(int l, int r)
{
	return uniform_int_distribution<int>(l,r)(rnd);
}

namespace hungeazy {

	void solve(void)
	{
		ull ans = 0;
		FOR(i,1,n) ans += (m-k)*val[i];
		// cout << ans << endl;
		FOR(i,1,n)
		{
			ull sum = 0;
			FOR(j,1,m) sum += num[j][s[i][j]-'A'];
			sum -= m*val[i];
			// cout << sum << " " << ans-(m-k)*val[i] << endl;
			if (sum == ans-(m-k)*val[i])
			{
				cout << i;
				exit(0);
			}
		}
		cout << -1;
	}
	
}

signed main()
{
    fast;
    if (fopen(name".inp","r"))
    {
    	freopen(name".inp","r",stdin);
    	freopen(name".out","w",stdout);
    }
    rnd.seed(time(0));
    cin >> n >> m >> k;
    FOR(i,1,n) val[i] = randRange(1,1e6);
    FOR(i,1,n)
    {
    	cin >> s[i];
    	s[i] = '#'+s[i];
    	FOR(j,1,m) num[j][s[i][j]-'A'] += val[i];
    }
    hungeazy::solve();
    return 0;
}
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