#include <bits/stdc++.h>
#define pb push_back
using namespace std;
const int N = 1e5 + 10;
int n, t, cnt, ans;
bool mark[N];
vector<int> G[N];
unordered_map<string, int> mp;
void dfs(int v) {
mark[v] = true;
cnt++;
int u = G[v][0];
while (!mark[u]) cnt++, mark[u] = true, u = G[u][0];
return;
}
void solve() {
bool a[21][21] = {};
int dp[(1 << n) + 10];
memset(dp, 63, sizeof dp);
for (int i = 0; i < n; i++) {
string u, v;
cin >> u >> v;
if (mp[u] == 0) mp[u] = ++t;
if (mp[v] == 0) mp[v] = ++t;
a[mp[u]][mp[v]] = true;
}
dp[0] = 0;
for (int mask = 1; mask < (1 << n); mask++)
for (int lg = 0; lg < n; lg++)
if (mask & (1 << lg))
for (int lg1 = 0; lg1 < n; lg1++)
if (mask & (1 << lg1) && lg1 ^ lg)
dp[mask] = min(dp[mask], dp[mask ^ (1 << lg) ^ (1 << lg1)] - a[lg + 1][lg1 + 1] - a[lg1 + 1][lg + 1] + 2);
cout << dp[(1 << n) - 1] << '\n';
}
int main() {
ios:: sync_with_stdio(0), cin.tie(0);
cin >> n;
if (n & 1) return cout << "-1\n", 0;
if (n <= 20) {
solve();
return 0;
}
for (int i = 0; i < n; i++) {
string u, v;
cin >> u >> v;
if (mp[u] == 0) mp[u] = ++t;
if (mp[v] == 0) mp[v] = ++t;
G[mp[u]].pb(mp[v]);
}
for (int i = 1; i <= n; i++)
if (!mark[i]) {
dfs(i);
if (cnt ^ 2) ans += (cnt + 1) / 2;
cnt = 0;
}
cout << ans << '\n';
}
