#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
#define name "main"
const int N=(int)1e5;
int deg[N+2],to[N+2],u[N+2],v[N+2],n;
string s1[N+2],s2[N+2];
vector<string>nen;
namespace sub1{
bool check(){
return n<=20;
}
#define BIT(mask,x) (((mask)>>(x))&(1))
#define MASK(x) ((1)<<(x))
const int NN=20;
int dp[MASK(NN)];
void main_code(){
memset(dp,0x3f,sizeof dp);
dp[0]=0;
int INF=dp[1];
for(int mask=0;mask<MASK(n);++mask){
for(int i=1;i<=n;++i){
if (BIT(mask,i-1)) continue;
for(int k=i+1;k<=n;++k){
if (!BIT(mask,k-1)){
dp[mask|MASK(i-1)|MASK(k-1)]=min(dp[mask|MASK(i-1)|MASK(k-1)],dp[mask]+(to[i]!=k)+(to[k]!=i));
}
}
}
}
if (dp[MASK(n)-1]==INF) dp[MASK(n)-1]=-1;
cout<<dp[MASK(n)-1];
}
}
namespace sub2{
bool check(){
for(int i=1;i<=n;++i){
if (deg[i]!=0) return false;
}
return true;
}
const int NN=(int)3e5;
bool used[NN+2];
void main_code(){
int ans=0;
for(int i=1;i<=n;++i){
int cnt=0;
for(int j=i;!used[j];j=to[j]) {
used[j]=true;
++cnt;
}
if (cnt!=2) ans+=(cnt+1)/2;
}
cout<<ans<<'\n';
}
}
int32_t main(){
ios::sync_with_stdio(0);
cin.tie(0); cout.tie(0);
cin>>n;
if (n%2==1){
cout<<-1;
exit(0);
}
for(int i=1;i<=n;++i){
cin>>s1[i]>>s2[i];
nen.push_back(s1[i]);
nen.push_back(s2[i]);
}
sort(nen.begin(),nen.end());
nen.resize(unique(nen.begin(),nen.end())-nen.begin());
for(int i=1;i<=n;++i){
u[i]=upper_bound(nen.begin(),nen.end(),s1[i])-nen.begin();
v[i]=upper_bound(nen.begin(),nen.end(),s2[i])-nen.begin();
to[u[i]]=v[i];
deg[v[i]]++;
}
if (sub1::check()){
sub1::main_code();
exit(0);
}
if (sub2::check()){
sub2::main_code();
exit(0);
}
exit(0);
}
