#include <bits/stdc++.h>
using namespace std;
const int MX = 2e5 + 10;
struct dsu{
int p[MX], cc[MX];
void init(){
for(int i = 0; i < MX; i++) p[i] = i, cc[i] = 1;
}
int f(int x){
if(p[x] == x) return x;
else return p[x] = f(p[x]);
}
void Join(int u, int v){
int fu = f(u), fv = f(v);
if(fu == fv) return;
p[fu] = fv;
cc[fv] += cc[fu];
}
} D;
int N, a[MX], b[MX], nx[MX], tim = 0;
map<string, int> mp;
bool dead[MX];
int main(){
ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
int ans = 0;
cin >> N;
for(int i = 0; i < N; i++){
string s, t; cin >> s >> t;
if(!mp.count(s)) mp[s] = ++tim;
if(!mp.count(t)) mp[t] = ++tim;
a[i] = mp[s], b[i] = mp[t];
nx[a[i]] = b[i];
}
if(N % 2 == 1){
cout << -1 << '\n';
return 0;
}
for(int i = 1; i <= N; i++){
if(nx[i] == i){
dead[i] = true;
ans++;
continue;
}
if(!dead[i] && !dead[nx[i]] && nx[nx[i]] == i){
dead[i] = true;
dead[nx[i]] = true;
}
}
D.init();
for(int i = 1; i <= N; i++){
if(!dead[i] && !dead[nx[i]]) D.Join(i, nx[i]);
}
for(int i = 1; i <= N; i++){
if(dead[i]) continue;
if(D.f(i) == i){
int curr = D.cc[i];
ans += (curr + 1) / 2;
}
}
cout << ans << '\n';
}
