#include<bits/stdc++.h> using namespace std; #define int long long const int maxn = 1e5 + 5; vector<int> g[maxn]; signed main(){ ios_base::sync_with_stdio(false); cin.tie(0); cout.tie(0); int n; cin >> n; int id = 1; map<string, int> mp; for(int i = 1; i <= n; i++){ string s, t; cin >> s >> t; if(!mp.count(s)) mp[s] = id++; if(!mp.count(t)) mp[t] = id++; int u = mp[s], v = mp[t]; g[u].push_back(v); } if(n & 1){ cout << -1; return 0; } vector<int> deg(n + 1), c2(n + 1), vis(n + 1), f(n + 1); for(int i = 1; i <= n; i++) if(g[i].size()) deg[g[i][0]]++; for(int i = 1; i <= n; i++){ if(g[i].empty()) continue; int v = g[i][0]; if(g[v].size() && g[v][0] == i && i != v) c2[i] = 1; } queue<int> q; for(int i = 1; i <= n; i++) if(!deg[i]) q.push(i); int kept = 0; while(q.size()){ int u = q.front(); q.pop(); int v = g[u][0]; if(!vis[u]){ if(!vis[v] && !c2[v] && u != v){ vis[u] = 1; vis[v] = 1; kept++; } } deg[v]--; if(deg[v] == 0) q.push(v); } for(int i = 1; i <= n; i++){ if(vis[i]) continue; if(c2[i]){ int v = g[i][0]; vis[i] = vis[v] = 1; kept += 2; }else{ int cur = i, len = 0; while(!vis[cur]){ vis[cur] = 1; len++; cur = g[cur][0]; } kept += len / 2; } } cout << n - kept; }