#include <bits/stdc++.h>
using namespace std;
// macros
typedef long long ll;
typedef long double ld;
typedef pair<int, int> ii;
typedef pair<ll, ll> lll;
typedef tuple<int, int, int> iii;
typedef vector<int> vi;
typedef vector<ii> vii;
typedef vector<iii> viii;
typedef vector<ll> vll;
typedef vector<lll> vlll;
#define REP(a,b,c) for(int a=int(b); a<int(c); a++)
#define RE(a,c) REP(a,0,c)
#define RE1(a,c) REP(a,1,c+1)
#define REI(a,b,c) REP(a,b,c+1)
#define REV(a,b,c) for(int a=int(c-1); a>=int(b); a--)
#define FOR(a,b) for(auto& a : b)
#define all(a) a.begin(), a.end()
#define INF 1e18
#define EPS 1e-9
#define pb push_back
#define popb pop_back
#define fi first
#define se second
#define sz size()
mt19937_64 rng(chrono::steady_clock::now().time_since_epoch().count());
// input
template<class T> void IN(T& x) {cin >> x;}
template<class H, class... T> void IN(H& h, T&... t) {IN(h); IN(t...); }
// output
template<class T1, class T2> void OUT(const pair<T1,T2>& x);
template<class T> void OUT(const vector<T>& x);
template<class T> void OUT(const T& x) {cout << x;}
template<class H, class... T> void OUT(const H& h, const T&... t) {OUT(h); OUT(t...); }
template<class T1, class T2> void OUT(const pair<T1,T2>& x) {OUT(x.fi,' ',x.se);}
template<class T> void OUT(const vector<T>& x) {RE(i,x.size()) OUT(i==0?"":" ",x[i]);}
template<class... T> void OUTL(const T&... t) {OUT(t..., "\n"); }
template<class H> void OUTLS(const H& h) {OUTL(h); }
template<class H, class... T> void OUTLS(const H& h, const T&... t) {OUT(h,' '); OUTLS(t...); }
//===================//
// Added libraries //
//===================//
//===================//
//end added libraries//
//===================//
void program();
int main() {
ios_base::sync_with_stdio(false);
cin.tie(NULL);
cout.tie(NULL);
program();
}
//===================//
// begin program //
//===================//
const int MX = 5e5;
int n;
string t[MX];
map<string,int> mp;
int nxt[MX];
int in[MX];
set<int> pre[MX];
void program() {
IN(n);
RE(i,n) {
string s; IN(s,t[i]);
mp[s] = i;
}
if(n%2) {
OUTL(-1);
return;
}
RE(i,n) nxt[i] = mp[t[i]];
RE(i,n) in[nxt[i]]++;
RE(i,n) pre[nxt[i]].insert(i);
queue<int> q;
RE(i,n) if(in[i]==0) q.push(i);
set<int> nRem;
RE(i,n) nRem.insert(i);
int remove = 0;
int ans = 0;
while(!nRem.empty()) {
int u=0;
if(!q.empty()) {
u = q.front(), q.pop();
} else {
u = *nRem.begin();
}
int v=nxt[u];
nRem.erase(u);
if(!nRem.count(v)){
ans++;
continue;
}
nRem.erase(v);
if(nxt[v] == u) continue;
ans++;
in[nxt[v]]--;
if(in[nxt[v]] == 0) q.push(nxt[v]);
}
OUTL(ans);
}
