#include<bits/stdc++.h>
using namespace std;
#define int long long
const int maxn = 1e5 + 5, inf = 1e9;
vector<int> g[maxn], rev[maxn];
int vis[maxn], f[maxn];
void dfs(int u){
vis[u] = 1;
for(int v: g[u]) if(!vis[v]) dfs(v);
}
void dfs2(int u){
vis[u] = 1;
for(int v: rev[u]){
if(vis[v] || v == u) continue;
dfs2(v);
if(!f[v] && !f[u]) f[u] = f[v] = 1;
}
}
signed main(){
ios_base::sync_with_stdio(false);
cin.tie(0); cout.tie(0);
int n; cin >> n;
vector<vector<int>> has;
if(n <= 20){
has = vector<vector<int>>(22, vector<int>(22));
}
int id = 1;
map<string, int> mp;
vector<int> ok(n + 1), skibidivertice;
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);
ok[v] = 1;
if(n <= 20) has[u - 1][v - 1] = 1;
if(u == v) skibidivertice.push_back(u);
}
bool sub2 = true;
for(int i = 1; i <= n; i++) sub2 &= (ok[i]);
if(n & 1){
cout << -1;
return 0;
}
if(n <= 20){
vector<int> dp(1 << n, inf);
dp[0] = 0;
for(int mask = 0; mask < (1 << n); mask++){
if(__builtin_popcount(mask) % 2) continue;
for(int i = 0; i < n; i++){
if(((mask >> i) & 1) == 1 || dp[mask] == inf) continue;
for(int j = 0; j < n; j++){
if(((mask >> j) & 1) == 0){
int nm = mask ^ (1 << i) ^ (1 << j);
dp[nm] = min(dp[nm], dp[mask] + (has[i][j] == 0) + (has[j][i] == 0));
}
}
}
}
cout << dp[(1 << n) - 1];
}else{
vector<int> deg(n + 1), c2(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();
if(vis[u]) continue;
int v = g[u][0];
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];
if(!vis[v]){
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;
}
}
