#include <bits/stdc++.h>
#define pb push_back
#define f first
#define sc second
using namespace std;
typedef long long int ll;
typedef string str;
int n;
vector<vector<int>> v;
vector<int> succ;
vector<bool> bl;
pair<int, int> cycle;
bool cycle_type;
void dfs_cycle(int nd, int ss){
if(bl[nd]){
cycle = {ss, nd};
return;
}
bl[nd] = 1;
unordered_map<int, int> mp;
for(int x: v[nd]) if(x != ss) dfs_cycle(x, nd), mp[x]++;
for(auto [x, c]: mp) if(c > 1){
cycle = {nd, x};
if(nd != x) cycle_type = 1;
}
}
int mc = 0;
bool dfs(int nd, int ss, bool type){
bool s = 0;
for(int x: v[nd])
if(x != ss && (nd != cycle.f || x != cycle.sc) && (nd != cycle.sc || x != cycle.f))
s|=dfs(x, nd, type);
if(ss == -1 && cycle_type) dfs(cycle.sc, nd, type);
if(type){
if(nd == cycle.f) return 0;
if(nd == cycle.sc){
mc++;
return 0;
}
}
if(s) mc++;
return !s;
}
int main(){
ios_base::sync_with_stdio(0);
cin.tie(0);
cin >> n;
if(n%2 != 0){
cout << "-1\n";
exit(0);
}
v.assign(n, {});
succ.assign(n, -1);
bl.assign(n, 0);
unordered_map<str, int> mp;
int in = 0;
for(int i = 0; i < n; i++){
str a, b; cin >> a >> b;
if(mp.find(a) == mp.end()) mp[a] = in, in++;
if(mp.find(b) == mp.end()) mp[b] = in, in++;
succ[mp[a]] = mp[b];
v[mp[a]].pb(mp[b]);
v[mp[b]].pb(mp[a]);
}
int ans = 0;
for(int i = 0; i < n; i++){
if(bl[i]) continue;
int mx = 0;
cycle_type = 0;
dfs_cycle(i, -1);
mc = 0;
dfs(cycle.f, -1, 0);
mx = max(mx, mc);
if(cycle.f != cycle.sc){
mc = 0;
dfs(cycle.f, -1, 1);
mx = max(mx, mc+cycle_type);
}
ans+=mx;
}
ans = n-ans;
cout << ans << "\n";
}
