/*
Note :
Link :
Boka hoye gelum...
*/
#include "bits/stdc++.h"
#define sp <<" "
#define el <<"\n"
#define S second
#define F first
#define pb push_back
#define all(ar) ar.begin(),ar.end()
#define pii pair<ll,ll>
using namespace std;
using ll = long long;
using ld = long double;
const ll mod = 1000000007;
const ll oo = 3372036000000000;
const ll si = 100005;
int cru[si], phead = -1,tin[si], psi = 0;
bool done[si], pol[si],vis[si];
map<string,int> rec;
vector<int> tps,head;
int ans = 0;
void dfs(int v,int tim){
if(tin[v] == tim){
phead = v;
return;
}
tin[v] = tim;
if(cru[v] != v && (tin[cru[v]] == -1 || tin[cru[v]] == tim)) dfs(cru[v], tim);
if(phead != -1){
pol[v] = 1;
psi++;
}
if(phead == v){
phead = -1;
head.pb(v);
}
return;
}
void top_sort(int v){
vis[v] = 1;
if(!vis[cru[v]]) top_sort(cru[v]);
tps.pb(v);
}
signed main(){
ios_base::sync_with_stdio(false);
cin.tie(NULL);
/*
#ifndef ONLINE_JUDGE
freopen("input.txt","r",stdin);
freopen("output.txt","w",stdout);
#endif
//*/
int n,cur = 0; cin>>n;
memset(done,false,sizeof(done));
for(int i = 0; i < n; i++){
string a,b; cin>>a>>b;
if(!rec.count(a)) rec[a] = cur++;
if(!rec.count(b)) rec[b] = cur++;
cru[rec[a]] = rec[b];
}
if(n % 2){
cout<<-1 el; return 0;
}
for(int i = 0; i < n; i++){
int des = cru[i];
if(i != des && cru[des] == i){
done[i] = done[des] = 1;
}
}
memset(vis,0,sizeof(vis));
for(int i = 0; i < n; i++) if(!vis[i] && !done[i]) top_sort(i);
reverse(all(tps));
memset(tin,-1,sizeof(tin));
memset(pol,0,sizeof(pol));
for(int i: tps) if(!done[i] && tin[i] == -1) dfs(i,i);
for(int i: tps) if(!done[i] && cru[i] != i && !pol[i] && !done[cru[i]]){
done[i] = done[cru[i]] = 1;
ans += 1;
}
for(int x: head){
psi = 0; bool xe = 1;
vector<int> mamu;
for(int r = x; r != x || xe ; r = cru[r]){
xe = 0;
psi++;
mamu.pb(r);
}
if(psi % 2 == 0){
ans += psi/2;
for(int r: mamu) done[r] = 1;
}
}
for(int i: tps) if(!done[i] && cru[i] != i && !done[cru[i]]){
done[i] = done[cru[i]] = 1;
ans += 1;
}
for(int i: tps) if(!done[i]) ans += 1;
cout<<ans el;
return 0;
}
/*
check for:
*missed a break, continue ?
*corner cases, n = 1?
*overflow.
*unnecessary logics.
*impreciseness caused by double or long double.
*/
