#include<bits/stdc++.h>
#define debug(x) cerr << #x << " " << x << "\n"
#define debugs(x) cerr << #x << " " << x << " "
#pragma GCC optimize("Ofast")
#define int long long
using namespace std;
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
int range_rng(int l, int r)
{
return uniform_int_distribution<int>(l,r)(rng);
}
map<int,int> taken;
string s[4505];
int rndval[4505];
int coef[4505];
int sum[4][4505];
int sumhash[4][4505];
const int mod=1e9+7;
signed main()
{
/*
ifstream fin("arbore.in");
ofstream fout("arbore.out");
*/
ios_base::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
int n,m,k,i,j,kek,totalsum,hashsum,normalsum,is;
cin >> n >> m >> k;
totalsum=0;
for (i=1; i<=n; i++)
{
do
{
kek=range_rng(1,1e9);
}
while (taken[kek]);
taken[kek]=1;
rndval[i]=kek;
cin >> s[i];
for (j=0; j<s[i].size(); j++)
{
if (s[i][j]=='A')
{
sum[0][j+1]++;
sumhash[0][j+1]+=rndval[i];
}
if (s[i][j]=='C')
{
sum[1][j+1]++;
sumhash[1][j+1]+=rndval[i];
}
if (s[i][j]=='G')
{
sum[2][j+1]++;
sumhash[2][j+1]+=rndval[i];
}
if (s[i][j]=='T')
{
sum[3][j+1]++;
sumhash[3][j+1]+=rndval[i];
}
}
totalsum+=rndval[i]*k;
}
for (i=1;i<=n;i++)
{
hashsum=0;
normalsum=0;
for (j=0;j<s[i].size();j++)
{
if (s[i][j]=='A')
{
normalsum+=sum[1][j+1];
hashsum+=sumhash[1][j+1];
normalsum+=sum[2][j+1];
hashsum+=sumhash[2][j+1];
normalsum+=sum[3][j+1];
hashsum+=sumhash[3][j+1];
}
if (s[i][j]=='C')
{
normalsum+=sum[0][j+1];
hashsum+=sumhash[0][j+1];
normalsum+=sum[2][j+1];
hashsum+=sumhash[2][j+1];
normalsum+=sum[3][j+1];
hashsum+=sumhash[3][j+1];
}
if (s[i][j]=='G')
{
normalsum+=sum[0][j+1];
hashsum+=sumhash[0][j+1];
normalsum+=sum[1][j+1];
hashsum+=sumhash[1][j+1];
normalsum+=sum[3][j+1];
hashsum+=sumhash[3][j+1];
}
if (s[i][j]=='T')
{
normalsum+=sum[0][j+1];
hashsum+=sumhash[0][j+1];
normalsum+=sum[1][j+1];
hashsum+=sumhash[1][j+1];
normalsum+=sum[2][j+1];
hashsum+=sumhash[2][j+1];
}
}
if (normalsum==(n-1)*k && hashsum==(totalsum-rndval[i]*k))
{
is=i;
break;
}
}
cout << is;
return 0;
}
