#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef vector<int> vi;
typedef vector<vi> vii;
typedef pair<int,int> pi;
typedef vector<pi> pii;
typedef tuple<int,int,int> ti;
typedef vector<ll> li;
typedef vector<li> lii;
#define REP(i,a,b) for(int i=a;i<b;i++)
#define F first
#define S second
#define PB push_back
#define LSOne(s) ((s)&(-s))
#define all(x) x.begin(),x.end()
ll INF=1000000000000000010;
int inf=1e9+10;
ll M=1e9+7;
int n;
map<string,int> mp;
vi a,arr,vis;
void dfs(int x){
arr.PB(x);
vis[x]=1;
if(!vis[a[x]])dfs(a[x]);
}
int main() {
// ios::sync_with_stdio(0);
// cin.tie(0);
// freopen("test_input.txt", "r", stdin);
// freopen("test_output.txt", "w", stdout);
cin>>n;
a.resize(n);vis.resize(n,0);
int tmp=1;
vi ord,deg(n,0);
REP(i,0,n){
string x,y;cin>>x>>y;
if(mp[x]==0)mp[x]=tmp++;
if(mp[y]==0)mp[y]=tmp++;
a[mp[x]-1]=mp[y]-1;
if(mp[x]!=mp[y])deg[mp[y]-1]++;
// cout<<mp[x]-1<<" "<<mp[y]-1<<"\n";
}
if(n%2){
cout<<-1;
return 0;
}
REP(i,0,n)if(a[i]!=i&&a[i]!=-1&&i==a[a[i]]){
a[a[i]]=-1;
a[i]=-1;
}
REP(i,0,n)if(a[i]!=-1&&a[a[i]]==-1)a[i]=i;
queue<int> q;
REP(i,0,n)if(deg[i]==0&&a[i]!=-1){
q.push(i);
ord.PB(i);
}
while(!q.empty()){
int x=q.front();
deg[a[x]]--;
if(deg[a[x]]==0){
q.push(a[x]);
ord.PB(a[x]);
}
q.pop();
}
int ans=0;
int sz=ord.size();
vi ord2;
REP(i,0,sz)if(!vis[ord[i]]){
ans++;
vis[a[ord[i]]]=1;
vis[ord[i]]=1;
deg[a[ord[i]]]--;
if(deg[a[ord[i]]]==0)ord2.PB(a[a[ord[i]]]);
}
sz=ord2.size();
REP(i,0,sz)if(vis[ord2[i]]==0&&a[ord2[i]]!=-1){
dfs(ord2[i]);
int k=arr.size();
ans+=k/2+k%2;
arr.clear();
}
REP(i,0,n)if(vis[i]==0&&a[i]!=-1){
dfs(i);
int k=arr.size();
ans+=k/2+k%2;
arr.clear();
}
cout<<ans;
}
