#include <bits/stdc++.h>
#define nl "\n"
#define no "NO"
#define yes "YES"
#define fi first
#define se second
#define vec vector
#define task "main"
#define _mp make_pair
#define ii pair<int, int>
#define sz(x) (int)x.size()
#define all(x) x.begin(), x.end()
#define evoid(val) return void(std::cout << val)
#define FOR(i, a, b) for(int i = (a); i <= (b); ++i)
#define FOD(i, b, a) for(int i = (b); i >= (a); --i)
#define unq(x) sort(all(x)); x.resize(unique(all(x)) - x.begin())
using namespace std;
template<typename U, typename V> bool maxi(U &a, V b) {
if (a < b) { a = b; return 1; } return 0;
}
template<typename U, typename V> bool mini(U &a, V b) {
if (a > b or a == -1) { a = b; return 1; } return 0;
}
const int N = (int)2e5 + 9;
const int mod = (int)1e9 + 7;
void prepare(); void main_code();
int main() {
ios::sync_with_stdio(0); cin.tie(0); cout.tie(0);
if (fopen(task".inp", "r")) {
freopen(task".inp", "r", stdin);
freopen(task".out", "w", stdout);
}
const bool MULTITEST = 0; prepare();
int num_test = 1; if (MULTITEST) cin >> num_test;
while (num_test--) { main_code(); }
}
void prepare() {};
int n, g[N];
int dp[21][1 << 20];
void subtask1() {
memset(dp, -1, sizeof dp); dp[0][0] = 0;
FOR(i, 0, n - 1)
FOR(mask, 0, (1 << n) - 1) if (dp[i][mask] != -1) {
if (1 & (mask >> i)) {
mini(dp[i + 1][mask], dp[i][mask]);
continue ;
}
FOR(k, 0, n - 1) if ( k != i and (1 & (mask >> k)) == 0 ) {
mini(dp[i + 1][mask | (1 << k) | (1 << i)],
dp[i][mask] + (g[i + 1] != k + 1)
+ (g[k + 1] != i + 1));
}
}
cout << dp[n][ (1 << n) - 1 ];
}
int deg[N], used[N], vis[N];
void main_code() {
cin >> n; map<string, int> mp;
if (n % 2 == 1) evoid(-1);
for(int i = 1, cnt = 0; i <= n; ++i) {
string x, y; cin >> x >> y;
if (mp.find(x) == mp.end()) {
mp[x] = ++cnt;
}
if (mp.find(y) == mp.end()) {
mp[y] = ++cnt;
}
int a = mp[x], b = mp[y];
g[a] = b;
}
if (n <= 20)
return subtask1(), void();
FOR(i, 1, n) ++deg[g[i]];
bool sub2 = 1;
FOR(i, 1, n) sub2 &= (deg[i] == 1);
if (sub2) {
int res = 0;
FOR(i, 1, n) if (vis[i] == 0) {
int cnt = 1; vis[i] = 1;
for(int j = g[i]; j != i; j = g[j]) {
++cnt; vis[j] = 1;
}
if (cnt == 2) continue ;
res += (cnt + 1) / 2;
}
cout << res;
return ;
}
queue<int> q;
int res = 0;
FOR(i, 1, n) {
if (i == g[i]) {
++res; used[i] = 1; deg[i] == 0;
continue ;
}
if (deg[i] == 0) q.push(i), used[i] = 1;
}
while (!q.empty()) {
int u = q.front(); q.pop();
if (used[u]) {
--deg[g[u]];
if (deg[g[u]] == 0) {
q.push(g[u]);
continue ;
}
}
++res;
used[u] = 1;
if (used[g[u]] == 0) {
used[g[u]] = 1;
--deg[g[u]];
if (deg[g[u]] == 0) q.push(g[u]);
continue ;
} else {
--deg[g[u]];
if (deg[g[u]] == 0) q.push(g[u]);
}
}
cout << res;
}
/* Let the river flows naturally */
