#include <bits/stdc++.h>
#define fr(i, n, m) for(int i = (n); i < (m); i ++)
#define pb push_back
#define st first
#define nd second
#define pq priority_queue
#define all(x) begin(x), end(x)
using namespace std;
typedef long long ll;
typedef long double ld;
typedef pair<ll,ll> pii;
const ll inf = 1e18;
const int i_inf = 1e9;
const ll mod = 1e9+7;
mt19937 _rand(time(NULL));
const int mxn = 2e5;
int n, m, k;
bitset<4100> bit[4100];
vector<int> v;
int tested[4100][4100];
void solve(){
memset(tested, -1, sizeof(tested));
cin >> n >> m >> k;
fr(i, 0, n){
string s;
cin >> s;
fr(j, 0, m){
if(s[j] == 'A') bit[i][j] = 1;
}
}
fr(i, 0, n) v.pb(i);
shuffle(all(v), _rand);
for(auto a : v){
bool ok = true;
for(auto b : v){
if(a == b) continue;
if(tested[a][b] != -1){
if(tested[a][b] == 0){
ok = false;
break;
}
continue;
}
int d = (bit[a]^bit[b]).count();
if(d != k){
tested[a][b] = 0;
ok = false;
break;
}
else{
tested[a][b] = 1;
}
}
if(ok){
cout<<a+1<<endl;
return;
}
}
cout<<-1<<endl;
}
int main(){
ios_base::sync_with_stdio(false);
cin.tie(NULL);
cout.tie(NULL);
solve();
}
