#include<cstdio>
#include<iostream>
#include<string>
#define RI register int
#define CI const int&
using namespace std;
const int N=100005;
const int mod1=998244353,mod2=1e9+7;
struct Hasher
{
	int x,y;
	inline Hasher(CI X=0,CI Y=0)
	{
		x=X; y=Y;
	}
	friend inline bool operator == (const Hasher& A,const Hasher& B)
	{
		return A.x==B.x&&A.y==B.y;
	}
	friend inline Hasher operator + (const Hasher& A,const Hasher& B)
	{
		return Hasher((A.x+B.x)%mod1,(A.y+B.y)%mod2);
	}
	friend inline Hasher operator - (const Hasher& A,const Hasher& B)
	{
		return Hasher((A.x-B.x+mod1)%mod1,(A.y-B.y+mod2)%mod2);
	}
	friend inline Hasher operator * (const Hasher& A,const Hasher& B)
	{
		return Hasher(1LL*A.x*B.x%mod1,1LL*A.y*B.y%mod2);
	}
}pw[N],f[N][4];
int n,m,k; string s[N];
const Hasher seed=Hasher(31,131);
int main()
{
    ios::sync_with_stdio(0); cin.tie(0);
	cin>>n>>m>>k;
    pw[0]=Hasher(1,1); for (RI i=1;i<=n;++i) pw[i]=pw[i-1]*seed;
    Hasher all_f,all_k;
    for (RI i=1;i<=n;++i)
	{
		cin>>s[i];
		for (RI j=0;j<m;++j)
		f[j][s[i][j]-'A']=f[j][s[i][j]-'A']+pw[i];
	}
	for (RI i=0;i<m;++i) for (RI j=0;j<4;++j) all_f=all_f+f[i][j];
	for (RI i=1;i<=n;++i) all_k=all_k+pw[i]*Hasher(k,k);
	for (RI i=1;i<=n;++i)
	{
		Hasher cur=all_f;
		for (RI j=0;j<m;++j) cur=cur-f[j][s[i][j]-'A'];
		if (cur==all_k-pw[i]*Hasher(k,k)) return cout<<i,0;
	}
    return 0;
}