#include <bits/stdc++.h>
#define speed ios_base::sync_with_stdio(0); cin.tie(0)
#define all(x) (x).begin(),(x).end()
#define F first
#define S second
using namespace std;
typedef long long ll;
typedef double db;
typedef long double ldb;
typedef pair<int,int> pii;
typedef pair<ll,ll> pll;
const ll MAX=1e5+10,P=998244353;
const ll INF=0x3f3f3f3f,oo=0x3f3f3f3f3f3f3f3f;
int n;
vector<pair<string,string> > eg;
int G[MAX];
vector<int> cptG[MAX];
void dfs(int v,int p,vector<int>& sz) {
sz[v]=1;
for (int i:cptG[v]) {
if (i==p) continue;
dfs(i,v,sz);
sz[v]+=sz[i];
}
}
int dfs2(int v,int p,bool can,vector<int>& sz) {
// cout<<v<<" "<<p<<" "<<can<<"\n";
if (sz[v]==1) return can;
int ans=0;
int minn=INF;
int id=0;
if (can) {
for (int i:cptG[v]) {
if (i==p) continue;
if (sz[i]<minn) {
minn=sz[i];
id=i;
}
}
ans+=dfs2(id,v,0,sz)+1;
}
for (int i:cptG[v]) {
if (i==p) continue;
if (can and i==id) continue;
ans+=dfs2(i,v,1,sz);
}
return ans;
}
int main() {
speed;
cin>>n;
vector<string> dct;
for (int i=0;i<n;i++) {
string a,b;
cin>>a>>b;
eg.emplace_back(a,b);
dct.push_back(a);
dct.push_back(b);
}
sort(all(dct));
dct.resize(unique(all(dct))-dct.begin());
vector<int> lvd(n,0);
for (int i=0;i<n;i++) {
auto [a,b]=eg[i];
int va=lower_bound(all(dct),a)-dct.begin();
int vb=lower_bound(all(dct),b)-dct.begin();
G[va]=vb;
cptG[va].push_back(vb);
cptG[vb].push_back(va);
lvd[vb]++;
}
bool alllvd=true;
for (int i=0;i<n;i++) {
if (lvd[i]!=1) {
alllvd=false;
break;
}
}
if (n&1) {
cout<<"-1\n";
return 0;
}
if (n<=20) {
int ans=n;
for (int st=0;st<(1<<n);st++) {
bool flag=true;
vector<int> cnt(n,0);
for (int i=0;i<n;i++) {
if (st&(1<<i)) continue;
if (G[i]==i) {
flag=false;
break;
}
int tgt=G[i];
if (G[tgt]==i) continue;
if (st&(1<<tgt)) {
if (cnt[tgt]==0) {
cnt[tgt]++;
continue;
}
}
flag=false;
break;
}
// if (flag) cout<<st<<"\n";
if (flag) ans=min(ans,__builtin_popcount(st));
}
cout<<ans<<"\n";
} else if (alllvd) {
vector<bool> vis(n,0);
int ans=0;
for (int i=0;i<n;i++) {
if (vis[i]) continue;
int now=i;
int st=i;
int sz=1;
vis[i]=true;
while (G[now]!=st) {
// cout<<now<<" ";
sz++;
now=G[now];
vis[now]=true;
}
if (sz==2) continue;
ans+=(sz+1)>>1;
}
cout<<ans<<"\n";
} else {
vector<int> sz(n);
int ans=0;
for (int i=0;i<n;i++) {
if (G[i]==i) {
dfs(i,i,sz);
ans+=dfs2(i,i,1,sz);
}
}
cout<<ans<<"\n";
}
return 0;
}
