#include <bits/stdc++.h>
#pragma GCC optimize("O3,unroll-loops")
#pragma GCC target("avx2,bmi,bmi2,popcnt,lzcnt")
#define se second
#define pb push_back
#define fi first
#define fo(var, n) for(long long var=0;var<n;var++)
#define fore(var, n) for(long long var=n-1;var>=0;var--)
using namespace std;
using ll = long long;
int main()
{
ios_base::sync_with_stdio(0); cin.tie(0);
int tt = 1;
//cin>>tt;
while(tt--)
{
int n; cin>>n;
map<string, int> t;
int jj=0;
int cnt[n];
fo(i,n) cnt[i] = 0;
int tar[n];
fo(i,n)
{
string a, b; cin>>a>>b;
if(t.find(a) == t.end()) {t[a] = jj;jj++;}
if(t.find(b) == t.end()) {t[b] = jj;jj++;}
cnt[t[b]]++;
tar[t[a]] = t[b];
if(a == b) cnt[t[b]]--;
}
if(n&1)
{
cout<<"-1\n"; continue;
}
set<pair<int,int>> c;
fo(i,n) c.insert({cnt[i], i});
int ans=0, sad=0;
int hav[n];
fo(i,n) hav[i] = 0;
// fo(i,n) cout<<tar[i]<<" ";cout<<"\n";
fo(i, n) if(tar[i] != i && tar[tar[i]] == i){hav[i]=1; hav[tar[i]] = 1;}
fo(i,n)
{
auto x = c.begin();
int j = (*x).se;
c.erase(x);
//cout<<j<<" "<<cnt[j]<<" "<<ans<<"\n";
if(hav[j]) continue;
if(hav[tar[j]] || tar[j] == j)
{
ans++; hav[j]=1;
}
else
{
ans++; hav[j]=1; hav[tar[j]]=1;
if(c.count({cnt[tar[tar[j]]], tar[tar[j]]}))
{
c.erase({cnt[tar[tar[j]]], tar[tar[j]]});
cnt[tar[tar[j]]]--;
c.insert({cnt[tar[tar[j]]], tar[tar[j]]});
}
}
}
cout<<ans<<"\n";
}
return 0;
}
