#include<bits/stdc++.h>
using namespace std;
using ll=long long;
vector<ll> nxt(1e6,-1);
map<string,ll> mp;
map<string,string> nx;
int main(){
ll n;cin>>n;
if(n%2){cout << -1;return 0;}
for(int i=0;i<n;i++){
string a,b;cin>>a>>b;
mp[a]=i;
nx[a]=b;
}
vector<ll> indeg(n);
for(pair<string,string> p:nx){
nxt[mp[p.first]]=mp[p.second];
indeg[mp[p.second]]++;
}
vector<set<ll>> in(n+1);
vector<ll> taken(n);
for(ll i=0;i<n;i++){
if(nxt[nxt[i]]==i&&nxt[i]!=i)taken[i]=1;
else in[indeg[i]].insert(i);
}
vector<ll> alone;
ll res=0;
while(1){
if(in[0].size()==0){
if(in[1].size()==0)break;
else{
int i=*in[1].begin();
in[1].erase(i);
if(nxt[i]==i){alone.push_back(i);}
else{
in[1].erase(nxt[i]);
taken[i]=taken[nxt[i]]=1;
if(!taken[nxt[nxt[i]]]){
in[1].erase(nxt[nxt[i]]);
in[0].insert(nxt[nxt[i]]);
}
res++;}
}
}
else{
int i=*in[0].begin();
in[0].erase(i);
if(nxt[i]==i){alone.push_back(i);}
else{
if(taken[nxt[i]])alone.push_back(i);
else{
res++;
in[indeg[nxt[i]]].erase(nxt[i]);
taken[nxt[i]]=1;taken[i]=1;
if(!taken[nxt[nxt[i]]]){
in[indeg[nxt[nxt[i]]]].erase(nxt[nxt[i]]);
indeg[nxt[nxt[i]]]--;
in[indeg[nxt[nxt[i]]]].insert(nxt[nxt[i]]);
}
}
}
}
}
cout << res+alone.size();
return 0;
}
