#include <iostream>
#include <vector>
#include <map>
#include <set>
#include <queue>
#include <stack>
using namespace std;
typedef long long ll;
vector<set <ll>> idg(100005);
vector<set <ll>> odg(100005);
map <string, ll> mapa;
map <string, bool> ima;
queue <ll> samci;
vector <ll> outcast;
ll neparni = 0;
int main()
{
ll n;
cin >> n;
vector <string> prvi;
vector <string> drugi;
ll nodes = 0;
for (int i = 0; i < n; i++){
string a, b;
cin >> a >> b;
prvi.push_back(a), drugi.push_back(b);
if (!ima[a])
ima[a] = true, mapa[a] = nodes, nodes++;
if (!ima[b])
ima[b] = true, mapa[b] = nodes, nodes++;
}
if (n%2 == 1){
cout << "-1" << endl;
return 0;
}
for (int i = 0; i < n; i++)
odg[mapa[prvi[i]]].insert(mapa[drugi[i]]), idg[mapa[drugi[i]]].insert(mapa[prvi[i]]);
for (int i = 0; i < n; i++)
if (idg[i].size() == 0)
samci.push(i);
ll brop = 0;
//cout << "ok" << endl;
vector <bool> vis(100000, false);
for (int i = 0; i < n; i++){
ll j = *odg[i].begin();
if (i == j)
continue;
if (i == *odg[j].begin())
vis[i] = true, vis[j] = true;
}
while (samci.size()){
ll tren = samci.front();
vis[tren] = true;
samci.pop();
ll ljubav = *odg[tren].begin();
if (vis[ljubav]){
idg[ljubav].erase(tren);
outcast.push_back(tren);
continue;
}
ll njljubav = *odg[ljubav].begin();
vis[ljubav] = true;
odg[ljubav].erase(njljubav);
idg[njljubav].erase(ljubav);
odg[ljubav].insert(tren);
brop++;
if (idg[njljubav].size() == 0 && !vis[njljubav])
samci.push(njljubav);
idg[tren].insert(ljubav);
}
//cout << brop << endl;
//cout << "ok" << endl;
for (int i = 0; i < n; i++){
if (!vis[i]){
ll pocetak = i;
ll len = 1;
ll tren = *odg[i].begin();
vis[i] = true;
while (tren != pocetak)
vis[tren] = true, len++, tren = *odg[tren].begin();
if (len == 2)
continue;
if (len%2 == 0)
brop += len/2;
else
brop += len/2, neparni++;
}
}
ll rjes = brop + neparni + outcast.size();
cout << rjes << endl;
}
