#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
#define name "main"
const int N=(int)1e6;
vector<string>nen;
string s1[N+2],s2[N+2];
int deg[N+2],num[N+2],low[N+2],id[N+2],scc=0,timedfs=0;
int u[N+2],v[N+2],to[N+2],dp[N+2][2],n;
vector<int>ord,bag[N+2],ke[N+2];
bool used[N+2],del[N+2];
void buildup(int u){
num[u]=low[u]=++timedfs;
ord.push_back(u);
if (!num[to[u]]&&!del[to[u]]){
buildup(to[u]);
low[u]=min(low[u],low[to[u]]);
}
else if (!del[to[u]]) low[u]=min(low[u],num[to[u]]);
if (low[u]==num[u]){
int v;
++scc;
do{
v=ord.back();
ord.pop_back();
id[v]=scc;
del[v]=true;
bag[id[v]].push_back(v);
}while(v!=u);
}
}
void dfs(int u){
if (ke[u].size()==0){
dp[u][0]=0;
dp[u][1]=1;
return;
}
for(auto&v:ke[u]){
dfs(v);
dp[u][1]+=dp[v][0];
}
dp[u][1]++;
for(auto&v:ke[u]){
dp[u][0]=max(dp[u][0],dp[u][1]-1+max(0,dp[v][1]-dp[v][0]));
}
}
int32_t main(){
ios::sync_with_stdio(0);
cin.tie(0); cout.tie(0);
cin>>n;
if (n%2==1){
cout<<-1;
exit(0);
}
map<string,int> mp;
int cn=0;
for(int i=1;i<=n;++i){
string s1,s2; cin>>s1>>s2;
if (mp.find(s1)==mp.end()) mp[s1]=++cn;
if (mp.find(s2)==mp.end()) mp[s2]=++cn;
u[i]=mp[s1];
v[i]=mp[s2];
to[u[i]]=v[i];
}
for(int i=1;i<=n;++i) if (!num[i]) buildup(i);
for(int i=1;i<=scc;++i){
for(auto&u:bag[i]){
if (id[to[u]]!=i){
ke[id[to[u]]].push_back(i);
deg[i]++;
}
}
}
for(int i=1;i<=scc;++i) {
sort(ke[i].begin(),ke[i].end());
ke[i].resize(unique(ke[i].begin(),ke[i].end())-ke[i].begin());
}
int ans=0;
// for(int i=1;i<=n;++i){
// cout<<u[i]<<' '<<v[i]<<'\n';
// }
// for(int i=1;i<=n;++i) cout<<id[i]<<' '; cout<<'\n';
for(int i=1;i<=scc;++i){
if (deg[i]==0){
dfs(i);
if (bag[i].size()==2) ans+=2; else ans+=bag[i].size()/2;
if (bag[i].size()%2==0) ans+=dp[i][1]-1; else ans+=dp[i][0];
}
}
cout<<n-ans;
exit(0);
}
