#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const ll INF = 1e9+7;
const ll MOD = 998244353;
typedef pair<ll,ll> ii;
#define iii pair<ll,ii>
#define id pair<ll,vector<ll> >
#define f(i,a,b) for(ll i = a;i < b;i++)
#define pb push_back
#define vll vector<ll>
#define F first
#define S second
#define all(x) (x).begin(), (x).end()
///I hope I will get uprating and don't make mistakes
///I will never stop programming
///sqrt(-1) Love C++
///Please don't hack me
///@TheofanisOrfanou Theo830
///Think different approaches (bs,dp,greedy,graphs,shortest paths,mst)
///Stay Calm
///Look for special cases
///Beware of overflow and array bounds
///Think the problem backwards
///Training
vector<vector<ll> >adj,rev,adj2;
bool v[100005] = {0};
ll p[100005];
ll siz[100005] = {0};
ll dp[100005][3];
vector<ll>order;
void dfs(ll idx){
v[idx] = 1;
for(auto x:adj[idx]){
if(!v[x]){
dfs(x);
}
}
order.pb(idx);
}
void dfs2(ll idx,ll par){
v[idx] = 1;
p[idx] = par;
siz[par]++;
for(auto x:rev[idx]){
if(!v[x]){
dfs2(x,par);
}
}
}
ll solve(ll idx,ll c){
if(dp[idx][c] != -1){
return dp[idx][c];
}
ll ans;
if(c == 0 || c == 2){
ans = 1 ^ (c/2);
for(auto x:adj2[idx]){
ans += min(solve(x,0),solve(x,1));
}
}
else{
ans = 1;
ll sum = 0;
ll mini = INF;
for(auto x:adj2[idx]){
sum += min(solve(x,0),solve(x,1));
mini = min(mini,solve(x,2) - min(solve(x,0),solve(x,1)));
}
ans += sum + mini;
}
return dp[idx][c] = ans;
}
int main(void){
ll n;
cin>>n;
if(n % 2 == 1){
cout<<-1<<"\n";
return 0;
}
map<string,ll>mp;
ll pos = 1;
adj.assign(n+5,vector<ll>());
adj2.assign(n+5,vector<ll>());
rev.assign(n+5,vector<ll>());
vector<ii>ed;
f(i,0,n){
string a,b;
cin>>a>>b;
ll x,y;
if(!mp.count(a)){
mp[a] = pos;
pos++;
}
if(!mp.count(b)){
mp[b] = pos;
pos++;
}
x = mp[a];
y = mp[b];
adj[x].pb(y);
rev[y].pb(x);
ed.pb(ii(x,y));
}
f(i,1,n+1){
if(!v[i]){
dfs(i);
}
}
reverse(all(order));
f(i,0,n+5){
v[i] = 0;
}
ll ans = 0;
for(auto x:order){
if(!v[x]){
dfs2(x,x);
if(siz[x] != 2){
ans += siz[x] / 2;
}
}
}
ll co[n+5] = {0};
for(auto x:ed){
ll i = p[x.F],j = p[x.S];
if(siz[i] % 2 == 1 && siz[j] % 2 == 1 && i != j){
adj2[j].pb(i);
co[i]++;
}
}
memset(dp,-1,sizeof dp);
f(i,1,n+1){
if(!co[i] && siz[i]){
ans += min(solve(i,0),solve(i,1));
}
}
cout<<ans<<"\n";
}
