#include <bits/stdc++.h>
#define ll long long
using namespace std;
#define pii pair<ll, ll>
ll oo = 1e9 + 9;
int n, m, k;
const int MAX = 4e5 + 5, MOD = 1e9 + 7, LOGMAX = 25;
vector<int> g[MAX];
map<string, int> mp;
int par[MAX];
bool vis[MAX];
int sub[MAX];
int dp[MAX][2];
void dfs(int node, int p){
sub[node] = 1;
vis[node] = 1;
int sum = 0;
for(int to : g[node]){
if(p == to) continue;
dfs(to, node);
sub[node] += sub[to];
sum += dp[to][0];
}
dp[node][1] = sum + 1;
for(int to : g[node]){
if(p == to) continue;
dp[node][0] = max(dp[to][1] + (sum - dp[to][0]), dp[node][0]);
}
}
void solve(){
cin >> n;
int c = 1;
for(int i = 1; i <= n; i++){
string a, b; cin >> a >> b;
if(!mp[a]) mp[a] = c++;
if(!mp[b]) mp[b] = c++;
g[mp[b]].push_back(mp[a]);
par[mp[a]] = mp[b];
}
if(n & 1){
cout << "-1\n";
return;
}
int ans = 0;
for(int i = 1; i <= n; i++){
if(par[i] == i){
dfs(i, i);
ans += sub[i] - dp[i][0];
}
}
cout << ans << '\n';
}
signed main(){
int t = 1;
for(int i = 1; i <= t; i++){
solve();
}
}
