#include<bits/stdc++.h>
#define int long long
#define pb push_back
#define fast ios_base::sync_with_stdio(false), cin.tie(nullptr), cout.tie(nullptr);
#define MOD 1000000007
#define inf 1e18
#define fi first
#define se second
#define FOR(i,a,b) for(int i=a;i<=b;i++)
#define FORD(i,a,b) for(int i=a;i>=b;i--)
#define sz(a) ((int)(a).size())
#define endl '\n'
#define pi 3.14159265359
#define TASKNAME "differences"
using namespace std;
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; }
typedef pair<int,int> ii;
typedef pair<int,ii> iii;
typedef vector<int> vi;
const int MAXN = 1e5 + 9;
const int base = 744196469;

int n, m, k;
string str[MAXN];

int pw[MAXN], hashCode[MAXN][5];

int mul(int a, int b){
    return (a * b) % MOD;
}

int add(int a, int b){
    return (a + b) % MOD;
}

int sub(int a, int b){
    return ((a - b) % MOD + MOD) % MOD;
}
main()
{
    fast;
    if (fopen(TASKNAME".inp","r")){
        freopen(TASKNAME".inp","r",stdin);
        freopen(TASKNAME".out","w",stdout);
    }
    cin >> n >> m >> k;
    pw[0] = 1;
    int sumEqual = 0;
    FOR(i, 1, n){
        pw[i] = mul(pw[i - 1], base);
        sumEqual = add(sumEqual, mul(k, pw[i]));
    }

    FOR(i, 1, n){
        cin >> str[i];
        FOR(j, 0, m - 1){
            int c = str[i][j] - 'A';
            hashCode[j][c] = add(hashCode[j][c], pw[i]);
        }
    }

    FOR(i, 1, n){
        int needSum = sub(sumEqual, mul(k, pw[i]));

        int others = 0;
        FOR(j, 0, m - 1){
            int c = str[i][j] - 'A';
            FOR(k, 0, 3){
                if (c != k) others = add(others, hashCode[j][k]);
            }
        }

        if (others == needSum){
            cout << i << endl;
            exit(0);
        }
    }


}
/**
Warning:
Đọc sai đề???
Cận lmao
Code imple thiếu case nào không.
Limit.
**/