#include <bits/stdc++.h>
#define fr(i, n, m) for(int i = (n); i < (m); i ++)
#define pb push_back
#define st first
#define nd second
#define pq priority_queue
#define all(x) begin(x), end(x)
using namespace std;
typedef long long ll;
typedef long double ld;
typedef pair<ll,ll> pii;
const ll inf = 1e18;
const int i_inf = 1e9;
const ll mod = 1e9+7;
mt19937 _rand(time(NULL));
const int mxn = 2e5;
bool vis[mxn];
int n;
vector<int> g[mxn];
vector<int> tree[mxn];
int link[mxn];
unordered_map<string, int> mapa;
vector<int> r;
void mark(int u){
vis[u] = true;
for(auto e : g[u]){
if(vis[e]) continue;
tree[u].pb(e);
mark(e);
}
}
void find_roots(){
fr(i, 0, n){
if(vis[i]) continue;
int x = i;
vector<int> undo;
while(!vis[x]){
vis[x] = true;
undo.pb(x);
x = link[x];
}
int a = x;
vector<int> v;
do{
v.pb(a);
a = link[a];
}while(a != x);
for(auto u : undo) vis[u] = false;
for(auto u : v) vis[u] = true;
for(auto u : v){
r.pb(u);
mark(u);
}
}
}
int dp[mxn][2];
void dfs(int u){
for(auto e : tree[u]){
dfs(e);
}
int minsum = 0;
for(auto e : tree[u]){
minsum += min(dp[e][0], dp[e][1]);
}
dp[u][0] = dp[u][1] = minsum + 1;
for(auto e : tree[u]){
int tempsum = minsum - min(dp[e][0], dp[e][1]) + dp[e][0];
dp[u][1] = min(dp[u][1], tempsum);
}
}
void solve(){
cin >> n;
int c = 0;
fr(i, 0, n){
link[i] = i;
}
fr(i, 0, n){
string s;
cin >> s;
if(mapa[s] == 0){
mapa[s] = ++c;
}
int u = mapa[s]-1;
cin >> s;
if(mapa[s] == 0){
mapa[s] = ++c;
}
int v = mapa[s]-1;
link[u] = v;
g[v].pb(u);
}
if(n % 2){
cout<<-1<<endl;
return;
}
find_roots();
int ans = 0;
for(auto u : r){
dfs(u);
ans += min(dp[u][0], dp[u][1]);
}
cout<<ans<<endl;
}
int main(){
ios_base::sync_with_stdio(false);
cin.tie(NULL);
cout.tie(NULL);
solve();
}
