#include "bits/stdc++.h"
using namespace std;
typedef long long ll;
typedef vector<int>vi;
#define pb push_back
#define sz(v) (int)v.size()
typedef string str;
#define FOR(i,a,b) for(int i=a; i<b; i++)
void IO(){
#ifdef LOCAL
freopen("input.txt","r",stdin);
freopen("output.txt","w",stdout);
#endif
}
//------------------------------------------------------//
void ckmin(int &x, int y){x=min(x,y);}
const int MX=2e5;
int N;
vi out(MX),in(MX,0);
map<str,int>mp;
vi vis(MX,0);
int main(){
IO();
cin>>N;
int cnt=0;
FOR(i,0,N){
str s,ss; cin>>s>>ss;
if(!mp.count(s)) mp[s]=cnt++;
if(!mp.count(ss)) mp[ss]=cnt++;
out[mp[s]]=mp[ss];
if(s!=ss) in[mp[ss]]++;
}
if(N&1){
cout << -1 << endl;
return 0;
}
FOR(i,0,N) if(out[i]!=i && out[out[i]]==i) vis[i]=1,vis[out[i]]=1;
set<int>s;
FOR(i,0,N) if(in[i]==0) s.insert(i);
int ans=0;
while(sz(s)){
auto it=s.begin();
int u=*it;
s.erase(it);
if(out[u]==u) continue;
if(!vis[out[u]]){
if(out[out[u]]!=out[u]){
in[out[out[u]]]--;
if(in[out[out[u]]]==0) s.insert(out[out[u]]);
}
vis[u]=1; vis[out[u]]=1;
out[out[u]]=u;
ans++;
}
}
FOR(i,0,N){
int n=0,cur=i;
while(!vis[cur]){
vis[cur]=1; n++;
cur=out[cur];
}
if(n!=2) ans+=(n+1)/2;
}
cout << ans << endl;
}
