#include<bits/stdc++.h>
using namespace std;
#define int long long
const int maxn = 1e5 + 5, inf = 1e9;
vector<int> g[100005], rev[100005];
int vis[maxn], f[maxn], ans, sz;
void dfs(int u){
vis[u] = 1;
sz++;
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, ans++;
}
}
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 if(sub2){
int res = 0;
for(int i = 1; i <= n; i++){
if(!vis[i]){
sz = 0;
dfs(i);
if(sz != 2) res += (sz + 1) / 2;
}
}
cout << res;
}else{
for(int i = 1; i <= n; i++){
for(int x: g[i]) rev[x].push_back(i);
}
for(int x: skibidivertice) dfs2(x);
for(int i = 1; i <= n; i++) ans += !f[i];
cout << ans;
}
}
