#include <bits/stdc++.h>
#define fi first
#define se second
#define pb push_back
#define debug(...) fprintf(stderr, __VA_ARGS__)
using namespace std;

typedef long long ll;
const ll N = 100010, mod = 1e9 + 7, P = 13331;
int n, m, k;
ll f[27][N], g[N], pw[N], ans;
string s[N];
template<typename T>
T add(T a)  {return a;}
template<typename T, typename ...args>
T add(T a, args ...b)   {T tmp = a + add(b...); return tmp >= mod ? tmp - mod : tmp;}
int main()
{
    #ifdef Kelly
        freopen("in", "r", stdin);
        freopen("out", "w", stdout);
        freopen("err", "w", stderr);
    #endif
    ios::sync_with_stdio(false), cin.tie(0);
    cin >> n >> m >> k;
    for(int i = 1; i <= n; ++i) cin >> s[i];
    pw[0] = 1;
    for(int i = 1; i <= n; ++i) pw[i] = pw[i - 1] * P % mod;
    for(int i = 1; i <= n; ++i)
    {
        for(int j = 0; j < m; ++j)  f[s[i][j] - 'A'][j] = add(f[s[i][j] - 'A'][j], pw[i]);
        ans = add(ans, 1ll * pw[i] * k % mod);
    }
    for(int j = 0; j < m; ++j)
        for(int i = 0; i < 26; ++i) g[j] = add(g[j], f[i][j]);
    // cerr << "OK\n";
    for(int i = 1; i <= n; ++i)
    {
        ll sum = 0;
        for(int j = 0; j < m; ++j)
            sum = add(sum, g[j], mod - f[s[i][j] - 'A'][j]);
        if(sum == add(ans, mod - 1ll * pw[i] * k % mod))  {cout << i; return 0;}
    }
    cerr << "WA";
    return 0;
}