#define _CRT_SECURE_NO_WARNINGS
#include <iostream>
#include <iomanip>
#include <array>
#include <string>
#include <algorithm>
#include <cmath>
#include <set>
#include <map>
#include <unordered_set>
#include <unordered_map>
#include <vector>
#include <stack>
#include <queue>
#include <deque>
#include <bitset>
#include <list>
#include <iterator>
#include <numeric>
#include <complex>
#include <utility>
#include <random>
#include <cassert>
#include <fstream>
using namespace std;
mt19937 rnd(time(nullptr));
/* -------------------- Typedefs -------------------- */
typedef int itn;
typedef long long ll;
typedef unsigned long long ull;
typedef double db;
typedef float fl;
typedef long double ld;
/* -------------------- Usings -------------------- */
using vi = vector<int>;
using vll = vector<ll>;
using mii = map<int, int>;
using mll = map<ll, ll>;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
/* -------------------- Defines -------------------- */
#define ff first
#define ss second
#define pub push_back
#define pob pop_back
#define puf push_front
#define pof pop_front
#define mpr make_pair
#define yes cout<<"YES\n"
#define no cout<<"NO\n"
#define all(x) (x).begin(), (x).end()
#define USACO freopen("feast.in", "r", stdin); freopen("feast.out", "w", stdout);
/* -------------------- Constants -------------------- */
const int dx[8] = { -1, 0, 1, 0, -1, -1, 1, 1 };
const int dy[8] = { 0, -1, 0, 1, -1, 1, -1, 1 };
const int MAX = int(1e9 + 5);
const ll MAXL = ll(1e18) + 5ll;
const ll MOD = ll(1000000007);
const ll MOD2 = ll(998244353);
/* -------------------- Functions -------------------- */
void fastio() {
ios_base::sync_with_stdio(false);
cin.tie(0);
cout.tie(0);
}
void precision(int x) {
cout.setf(ios::fixed | ios::showpoint);
cout.precision(x);
}
ll gcd(ll a, ll b) {
if (a == 0 || b == 0) return 0;
while (b) {
a %= b;
swap(a, b);
}
return a;
}
ll lcm(ll a, ll b) {
return a / gcd(a, b) * b;
}
ll range_sum(ll a, ll b) {
if (a > b) return 0ll;
ll dif = a - 1, cnt = b - a + 1;
ll ans = ((b - a + 1) * (b - a + 2)) / 2;
ans += ((b - a + 1) * dif);
return ans;
}
string dec_to_bin(ll a) {
string s = "";
for (ll i = a; i > 0; ) {
ll k = i % 2;
i /= 2;
char c = k + 48;
s += c;
}
if (a == 0) {
s = "0";
}
reverse(all(s));
return s;
}
ll bin_to_dec(string s) {
ll num = 0;
for (int i = 0; i < s.size(); i++) {
num *= 2ll;
num += (s[i] - '0');
}
return num;
}
ll factorial_by_mod(ll n, ll mod) {
ll ans = 1;
ll num;
for (ll i = 1; i <= n; ++i) {
num = i % mod;
ans *= num;
ans %= mod;
}
return ans;
}
bool isPrime(ll a) {
if (a == 1) return false;
for (ll i = 2; i * i <= a; i++) {
if (a % i == 0) return false;
}
return true;
}
ll binpow(ll a, ll b) {
if (!a) return 0;
ll ans = 1;
while (b) {
if (b & 1) {
ans *= a;
}
b >>= 1;
a *= a;
}
return ans;
}
ll binpow_by_mod(ll a, ll b, ll mod) {
if (!a) return 0;
ll ans = 1;
while (b) {
if (b & 1) {
ans *= a;
ans %= mod;
}
b >>= 1;
a *= a;
a %= mod;
}
return ans;
}
/* -------------------- Solution -------------------- */
const int N = 100005;
int p[N], from[N], used[N], cyc[N], dep[N], rel[N];
set<int> g[N];
vi cycle;
vector<pii> tree_nodes;
int n, ans, azat;
void dfs_cyc(int u) {
used[u] = 1;
int to = p[u];
if (used[to]) {
if (to == u) {
cyc[u] = 1;
cycle = { u };
return;
}
int node = u;
while (1) {
cycle.pub(node);
cyc[node] = 1;
if (node == to) break;
node = from[node];
}
return;
}
from[to] = u;
dfs_cyc(to);
}
void dfs_tree(int u) {
used[u] = 1;
tree_nodes.pub({ dep[u],u });
for (auto to : g[u]) {
if (cyc[to]) continue;
dep[to] = dep[u] + 1;
dfs_tree(to);
}
}
void slv() {
cin >> n;
map<string, int> mp;
int cur = 0;
for (int i = 1; i <= n; ++i) {
string x, y;
cin >> x >> y;
if (!mp[x]) mp[x] = ++cur;
if (!mp[y]) mp[y] = ++cur;
int id1 = mp[x], id2 = mp[y];
g[id2].insert(id1);
p[id1] = id2;
}
if (n % 2) {
cout << -1;
return;
}
for (int u = 1; u <= n; ++u) {
if (used[u]) continue;
cycle.clear();
dfs_cyc(u);
reverse(all(cycle));
int sz = (int)cycle.size();
for (auto root : cycle) {
tree_nodes.clear();
dfs_tree(root);
sort(all(tree_nodes));
while (tree_nodes.size() > 1) {
int node = tree_nodes.back().ss;
if (rel[node] || rel[p[node]]) {
tree_nodes.pop_back();
continue;
}
if (cyc[p[node]] && sz == 2) {
tree_nodes.pop_back();
continue;
}
rel[node] = rel[p[node]] = 1;
++ans;
tree_nodes.pop_back();
}
}
if (sz == 1) continue;
if (sz == 2) {
rel[cycle[0]] = rel[cycle[1]] = 1;
continue;
}
int ind = 0;
for (int i = 1; i < sz; ++i) {
if (rel[cycle[i]]) {
ind = i;
break;
}
}
int id = ind + 1;
if (id == sz) id = 0;
while (id != ind) {
int node = cycle[id];
if (rel[node] || rel[p[node]]) {
++id;
if (id == sz) id = 0;
continue;
}
++ans;
rel[node] = rel[p[node]] = 1;
++id;
if (id == sz) id = 0;
++id;
if (id == sz) id = 0;
}
}
for (int u = 1; u <= n; ++u) {
if (!rel[u]) ++azat;
}
cout << ans + azat << '\n';
}
void cs() {
int tstc = 1;
//cin >> tstc;
while (tstc--) {
slv();
}
}
void precalc() {
return;
}
int main() {
fastio();
precalc();
//precision(0);
cs();
return 0;
}
