#include<bits/stdc++.h>
using namespace std;
#define int long long
struct unionfind{
vector<int> chef;
vector<int> cnt;
unionfind(int n){
chef.resize(n);
iota(chef.begin(), chef.end(), 0);
cnt.resize(n, 1);
}
int find(int a){
if(chef[a]==a) return a;
return chef[a]=find(chef[a]);
}
void unite(int a, int b){
if(find(a)==find(b)) return;
cnt[find(b)]+=cnt[find(a)];
chef[find(a)]=find(b);
}
};
void dfs(int v, vector<vector<int> >& adi, vector<bool>& vis, unionfind& ufind, vector<int>& postorder, bool first){
assert(!vis[v]);
vis[v]=true;
for(auto u: adi[v]){
if(!vis[u]){
dfs(u, adi, vis, ufind, postorder, first);
if(!first){
ufind.unite(u, v);
}
}
}
if(first){
postorder.push_back(v);
}
}
void kosaraju(int n, vector<vector<int> >& adi, vector<vector<int> >& rev, unionfind& ufind){
vector<int> postorder;
vector<bool> vis(n);
for(int v=0; v<n; v++){
if(!vis[v]) {
dfs(v, adi, vis, ufind, postorder, true);
}
}
vis.assign(n, false);
for(int v=n-1; v>=0; v--){
if(!vis[postorder[v]]) {
dfs(postorder[v], rev, vis, ufind, postorder, false);
}
}
}
void dfs_dp(int v, vector<vector<int> >& adi, vector<vector<int> >& dp, vector<bool>& vis, unionfind& ufind){
if(vis[v]) return;
vis[v]=true;
int add=0;
if(ufind.cnt[v]!=2){
add=(ufind.cnt[v]+1)/2;
}
int sum0=0;
for(auto u: adi[v]){
dfs_dp(u, adi, dp, vis, ufind);
sum0+=dp[u][0];
}
dp[v][1]=sum0;
dp[v][0]=sum0+add;
for(auto u: adi[v]){
if(ufind.cnt[v]%2==1){
dp[v][0]=min(dp[v][0], sum0-dp[u][0]+dp[u][1]+add);
}
}
//cout << v << " " << ufind.find(v) << " " << ufind.cnt[ufind.find(v)] << "\n";
//cout << dp[v][0] << " " << dp[v][1] << "\n";
}
int solve(int n, vector<vector<int> >& adi_alt, vector<vector<int> >& rev, vector<int>& par){
unionfind ufind(n);
kosaraju(n, adi_alt, rev, ufind);
vector<vector<int> > adi(n);
for(int u=0; u<n; u++){
int v=par[u];
if(v==-1) continue;
if(ufind.find(u)!=ufind.find(v)){
adi[ufind.find(v)].push_back(ufind.find(u));
}
}
/*for(int i=0; i<n; i++){
cout << i << ": " << ufind.find(i) << "\n";
}
for(int i=0; i<n; i++){
if(ufind.find(i)==i){
cout << i << ": ";
for(auto e: adi[i]){
cout << e << " ";
}
cout << "\n";
}
}*/
vector<bool> vis(n);
vector<vector<int> > dp(n, vector<int> (2));
int ans=0;
for(int v=0; v<n; v++){
if(vis[ufind.find(v)]) continue;
if(ufind.find(v)!=v || par[v]==-1){
dfs_dp(ufind.find(v), adi, dp, vis, ufind);
ans+=dp[ufind.find(v)][0];
}
}
return ans;
}
signed main(){
ios_base::sync_with_stdio(false);
cin.tie(0);
int n;
cin >> n;
int cnt=1;
map<string, int> ind;
vector<vector<int> > adi(n);
vector<vector<int> > rev(n);
vector<int> par(n, -1);
for(int i=0; i<n; i++){
string s, t;
cin >> s >> t;
if(ind[s]==0){
ind[s]=cnt;
cnt++;
}
if(ind[t]==0){
ind[t]=cnt;
cnt++;
}
int a=ind[s];
int b=ind[t];
if(a!=b){
adi[b-1].push_back(a-1);
rev[a-1].push_back(b-1);
par[a-1]=b-1;
}
}
if(n%2==1){
cout << -1 << "\n";
return 0;
}
cout << solve(n, adi, rev, par) << "\n";
}
