#include<bits/stdc++.h>
using namespace std;
const int N=2e5+5;
int a[N];
signed main(){
ios::sync_with_stdio(0); cin.tie(0);
int n; cin>>n;
map<string,int>m;
int d=1;
for(int i=0;i<n;i++){
string u,v; cin>>u>>v;
if(!m[u]) m[u]=d++;
if(!m[v]) m[v]=d++;
a[m[u]]=m[v];
}
int ans=0;
int vis[d]={0};
for(int i=1;i<d;i++){
int s=0;
int j=i;
while(!vis[j]){
vis[j]=1;
j=a[j]; s++;
}
ans+=(s+1)/2;
}
if(n%2) cout<<-1;
else cout<<ans;
}
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