/*
it could have been better :)
it will next time ;)
*/
#include <bits/stdc++.h>
using namespace std;
#define int long long
#define INF 1e18
#define f first
#define s second
#define pii pair<int, int>
#define vi vector<int>
const int MOD = 1'000'000'000 + 7;
void setIO(string name = "")
{
ios_base::sync_with_stdio(0);
cin.tie(0), cout.tie(0);
#ifdef LOCAL
freopen("inp.txt", "r", stdin);
freopen("out.txt", "w", stdout);
#else
if (!name.empty())
{
freopen((name + ".INP").c_str(), "r", stdin);
freopen((name + ".OUT").c_str(), "w", stdout);
}
#endif
}
// START OF DEBUG
#define db(val) "["#val" = "<<(val)<<"] "
#define print_op(...) ostream& operator<<(ostream& out, const __VA_ARGS__& u)
// for printing std::pair
template<class U, class V> print_op(pair<U, V>) {
return out << "(" << u.first << ", " << u.second << ")";
}
// for printing collection
template<class Con, class = decltype(begin(declval<Con>()))>
typename enable_if<!is_same<Con, string>::value, ostream&>::type
operator<<(ostream& out, const Con& con) {
out << "{";
for (auto beg = con.begin(), it = beg; it != con.end(); ++it)
out << (it == beg ? "" : ", ") << *it;
return out << "}";
}
// for printing std::tuple
template<size_t i, class T> ostream& print_tuple_utils(ostream& out, const T& tup) {
if constexpr(i == tuple_size<T>::value) return out << ")";
else return print_tuple_utils<i + 1, T>(out << (i ? ", " : "(") << get<i>(tup), tup);
}
template<class ...U> print_op(tuple<U...>) {
return print_tuple_utils<0, tuple<U...>>(out, u);
}
// END OF DEBUG
const int MAXN = 1e5;
int n;
int nxt[MAXN + 5];
map<string, int> mp;
namespace sub1 {
bool check() {
return n <= 20;
}
const int N = 20;
int dp[(1 << N)];
void solve() {
if(n & 1) {
cout << -1 << '\n';
return;
}
for(int i = 0; i < (1 << n); i++) dp[i] = INF;
dp[0] = 0;
for(int b = 0; b < (1 << n); b++) {
// cout << b << ' ' << __builtin_popcount(b) << '\n';
if(__builtin_popcount(b) % 2 == 1) continue;
for(int i = 0; i < n; i++) {
if(b & (1 << i)) continue;
for(int j = 0; j < n; j++) {
if(b & (1 << j)) continue;
int nb = b | (1 << i);
nb |= (1 << j);
int cnt = (nxt[i] != j) + (nxt[j] != i);
dp[nb] = min(dp[nb], dp[b] + cnt);
// cout << b << ' ' << nb << '\n';
// cout << dp[b] << ' ' << dp[nb] << '\n';
}
}
}
if(dp[(1 << n) - 1] == INF) dp[(1 << n) - 1] = -1;
cout << dp[(1 << n) - 1] << '\n';
}
}
void solve()
{
cin >> n;
vector<pair<string, string>> v;
vector<string> lst;
for(int i = 1; i <= n; i++) {
string s, t; cin >> s >> t;
v.push_back({s, t});
lst.push_back(s);
lst.push_back(t);
}
sort(lst.begin(), lst.end());
lst.erase(unique(lst.begin(), lst.end()), lst.end());
for(int i = 0; i < (int)lst.size(); i++) {
mp[lst[i]] = i;
}
// cout << mp << '\n';
for(auto it : v) {
nxt[mp[it.f]] = mp[it.s];
}
// for(int i = 0; i < n; i++) cout << nxt[i] << '\n';
sub1::solve();
}
signed main()
{
setIO();
int t = 1;
// cin >> t;
while (t--)
solve();
}
