#include<iostream>
#include<vector>
#include<map>
#include<set>
#include<cassert>
#include<unordered_map>
#include<unordered_set>
#include<functional>
#include<queue>
#include<stack>
#include<cstring>
#include<algorithm>
#include<cmath>
#include<sstream>
#include<iomanip>
#include<cstdio>
#include<cstdlib>
#include<numeric>
#include<random>
#include<chrono>
#include<bitset>
//#include<ext/pb_ds/assoc_container.hpp>
//#include<ext/pb_ds/tree_policy.hpp>
using namespace std;
//using namespace __gnu_pbds;
#define all(x) (x).begin(), (x).end()
#define pb push_back
#define eb emplace_back
#define xx first
#define yy second
#define sz(x) (int)(x).size()
#define gc getchar
#define IO ios_base::sync_with_stdio(false);cin.tie(0);cout.tie(0)
#define mp make_pair
#define ins insert
#define chkmn(x,y) (x)=min((x), (y))
#define chkmx(x,y) (x)=max((x), (y))
#define rep(i,a,b) for(int i=a;i<b;i++)
#ifndef ONLINE_JUDGE
# define LOG(x) (cerr << #x << " = " << (x) << endl)
#else
# define LOG(x) ((void)0)
#endif
using ll = long long;
using ull = unsigned long long ;
using ld = long double ;
using str = string;
//using ordered_set=tree<pair<int,int>, null_type, less<pair<int,int>>, rb_tree_tag, tree_order_statistics_node_update>;
const double PI=acos(-1);
const ll INF = 1LL<<62;
const ll MINF = -(1LL<<62);
template<typename T> T getint() {
T val=0;
char c;
bool neg=false;
while((c=gc()) && !(c>='0' && c<='9')) {
neg|=c=='-';
}
do {
val=(val*10)+c-'0';
} while((c=gc()) && (c>='0' && c<='9'));
return val*(neg?-1:1);
}
//mt19937 rng(chrono::steady_clock::now().time_since_epoch().count()); uniform_int_distribution<int>(0, n-1)(rng)
//CERC 2022F????
int conv(char c) {
if(c=='A') return 0;
if(c=='G') return 1;
if(c=='C') return 2;
if(c=='T') return 3;
assert(0);
}
int main() {
IO;
int n,m,k;
cin>>n>>m>>k;
array<ll,2> mul={3,5}, p={1,1};
array<ll,2> mod={(ll)1e9+7, (ll)1e9+9};
vector<array<array<ll,2>,4>> col(m);
vector<string> s(n);
array<ll,2> want{};
for(int i=0;i<n;++i) {
cin>>s[i];
for(int j=0;j<m;++j) {
int curr=conv(s[i][j]);
for(int k=0;k<4;++k) {
if(k!=curr) for(int l=0;l<2;++l) {
(col[j][k][l]+=p[l])%=mod[l];
}
}
}
for(int l=0;l<2;++l) (want[l]+=p[l]*k)%=mod[l];
for(int l=0;l<2;++l) (p[l]*=mul[l])%=mod[l];
}
//~ LOG(want[0]);
//~ LOG(want[1]);
p={1,1};
for(int i=0;i<n;++i) {
array<ll,2> sum{};
for(int j=0;j<m;++j) {
int curr=conv(s[i][j]);
for(int l=0;l<2;++l)
(sum[l]+=col[j][curr][l])%=mod[l];
}
//~ LOG(sum[0]);
//~ LOG(sum[1]);
bool ok=true;
for(int l=0;l<2;++l) {
ok&=(want[l]-k*p[l]-sum[l])%mod[l]==0;
}
if(ok) {
cout<<i+1<<"\n";
return 0;
}
for(int l=0;l<2;++l) (p[l]*=mul[l])%=mod[l];
}
assert(0);
return 0;
}
