#include <bits/stdc++.h>
using namespace std;
const int N = 5e5 + 5; 

int n, tot, to[N], deg[N], vis[N]; 
map<string, int> mp; 

int dfs(int x) {
    if (vis[x]) return 0; vis[x] = 1; 
    return dfs(to[x]) + 1; 
}

int main(void) {
    ios::sync_with_stdio(0); 
    cin >> n; int ans = 0; 
    if (n & 1) return cout << "-1\n", 0; 
    for (int i = 1; i <= n; ++i) {
        string s, t; cin >> s >> t; 
        int S = (mp[s] ? mp[s] : mp[s] = ++tot); 
        int T = (mp[t] ? mp[t] : mp[t] = ++tot); 
        if (S != T) to[S] = T; 
        else ++ans, vis[S] = 1; 
    }
    for (int i = 1; i <= n; ++i) if (i == to[to[i]]) vis[i] = vis[to[i]] = 1; 
    for (int i = 1; i <= n; ++i) if (!vis[i]) ++deg[to[i]]; 

    queue<int> q; 
    for (int i = 1; i <= n; ++i) if (!vis[i] && deg[i] == 0) q.push(i); 
    while (!q.empty()) {
        int u = q.front(), v = to[u]; q.pop(); ++ans; 
        if (!vis[v]) {
            vis[v] = 1; 
            if (!--deg[to[v]] && !vis[to[v]]) q.push(to[v]); 
        }
    }
    for (int i = 1; i <= n; ++i) if (!vis[i]) {
        int k = dfs(i); 
        if (k > 2) ans += (k + 1) / 2; 
    }
    cout << ans << '\n'; 
    return 0;
}