#include<bits/stdc++.h>
using namespace std;
#define int long long
const int maxn = 1e5 + 5;
vector<int> rev[maxn];
int par[maxn], deg[maxn], vis[maxn], match[maxn], ans;
void dfs(int u){
vis[u] = 1;
for(int v: rev[u]){
if(vis[v] || v == u || deg[v] > 0) continue;
dfs(v);
if(!match[v] && !match[u]){
match[u] = match[v] = 1;
ans++;
}
}
}
void dfscycle(int u){
vis[u] = 1;
for(int v: rev[u]){
if(deg[v] == 0) continue;
if(!vis[v]) dfscycle(v);
if(!match[v] && !match[u]){
match[u] = match[v] = 1;
ans++;
}
}
}
signed main(){
ios_base::sync_with_stdio(false);
cin.tie(0); cout.tie(0);
int n; cin >> n;
if(n & 1){
cout << -1;
return 0;
}
map<string, int> mp;
int id = 1;
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];
par[u] = v;
rev[v].push_back(u);
deg[v]++;
}
queue<int> q;
for(int i = 1; i <= n; i++) if(!deg[i]) q.push(i);
while(q.size()){
int u = q.front(); q.pop();
int v = par[u];
deg[v]--;
if(!deg[v]) q.push(v);
}
for(int i = 1; i <= n; i++){
if(!deg[i]) continue;
if(par[i] > i && par[par[i]] == i){//cycle size 2
match[i] = match[par[i]] = 1;
vis[i] = vis[par[i]] = 1;
dfs(par[i]);
}
dfs(i);
}
memset(vis, 0, sizeof(vis));
for(int i = 1; i <= n; i++) if(deg[i] && !vis[i] && match[i]) dfscycle(i);
for(int i = 1; i <= n; i++) if(deg[i] && !vis[i]) dfscycle(i);
for(int i = 1; i <= n; i++) ans += !match[i];
cout << ans;
}
