#include<bits/stdc++.h>
#define int long long
#define ss second
#define ff first
#define pb push_back
using namespace std;
const int mod=1e9+7;
const int mxn=5000;
const int b=13;
int n,m,k;
int s[mxn],sum,p[70][mxn],h[mxn];
string a[mxn];
signed main() {
ios_base::sync_with_stdio(false);
cin.tie(0);
cout.tie(0);
cin>>n>>m>>k;
sum=0;
for(int i=1;i<=n;i++)
{cin>>a[i];
a[i]=" "+a[i];
h[0]=0;
for(int j=1; j<=m; j++)
{h[j]=(h[j-1]*b%mod+a[i][j]-'A')%mod;
}
s[i]=h[m];
sum+=s[i];
for(int j=1; j<=m; j++)
{p[a[i][j]-'A'][j]+=s[i];
}
}
for(int i=1; i<=n; i++)
{int t=0;
for(int j=1; j<=m; j++)
{t+=p[a[i][j]-'A'][j]-s[i];
}
if(t==(sum-s[i])*(m-k)){cout<<i; return 0;}
}
}
