#include <bits/stdc++.h>
using namespace std;
using ll = long long;
const int N = 1e5+5;
const ll mod = 1e9+7;

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    ll p[N]; p[0] = 1;
    for (int i=1;i<=100000;++i) p[i] = p[i-1] * 998244353 % mod;
    int n,m,k; cin >> n >> m >> k;
    std::vector<string> s(n+1);
    std::vector<std::array<ll,4> > f(m+1);
    std::vector<ll> g(m+1);

    ll sum = 0;
    for (int i=1;i<=n;++i) {
        cin >> s[i];
        for (int j=0;j<m;++j) {
            (f[j][s[i][j]-'A'] += p[i] ) %= mod;
        }
        ( sum += k*p[i]) %= mod;
    }
    for (int i=0;i<m;++i) 
        for (int j=0;j<4;++j) (g[i] += f[i][j]) %= mod;
    
    for (int i=1;i<=n;++i) {
        ll sumg = 0;
        for (int j=0;j<m;++j) 
            (sumg += ((g[j]-f[j][s[i][j]-'A'])%mod+mod)%mod) %= mod;
        if (sumg%mod == ((sum-k*p[i])%mod+mod)%mod) {
            printf("%d\n",i);
            break;
        }
    }
    return 0;
}