#include <iostream>
#include <cstdio>
#include <algorithm>
using namespace std;
#define endl '\n'
#define pi pair<int, int>
#define f first
#define s second
#define ull unsigned long long
const int maxn = 4101, w = 4, b = 33;
int n, m, k;
int a[maxn][maxn];
ull p[maxn];
ull ss[maxn][w];
ull f, g;
int main(){
ios::sync_with_stdio(false);
cin.tie(NULL);
cin >> n >> m >> k;
p[0] = 1;
for(int i = 1; i <= n; i++){
string s;
cin >> s;
p[i] = b * p[i - 1];
f += p[i], g += k * p[i];
for(int j = 0; j < m; j++){
a[i][j] = (s[j] == 'A' ? 0 : s[j] == 'C' ? 1 : s[j] == 'G' ? 2 : 3);
ss[j][a[i][j]] += p[i];
}
}
for(int i = 1; i <= n; i++){
ull c = 0;
for(int j = 0; j < m; j++) c += f - ss[j][a[i][j]];
if(g == c + k * p[i]){
cout << i << endl;
return 0;
}
}
return 0;
}
