#include <bits/stdc++.h>
#define file_io freopen("input.txt", "r", stdin);freopen("output.txt", "w", stdout);
#define fast_io ios::sync_with_stdio(false);cin.tie(0);
#define what(x) cerr << #x << " is " << x << '\n';
#define kill(x) {cout << x << '\n'; return 0;}
#define all(x) (x).begin(), (x).end()
#define pii pair<int, int>
#define pb push_back
#define ll long long
#define F first
#define S second
const ll inf = 1e18, mod = 1e9 + 7, delta = 1e9 + 9, SQ = 450, P = 6065621;
using namespace std;
const ll N = 1e5 + 10, LG = 20;
vector<int> adj[N], r;
map<string, int> mp;
bitset<N> mark;
int dp[N][2];
void dfs (int v) {
mark[v] = true;
for (auto u: adj[v]) {
if (!mark[u]) {
dfs(u);
dp[v][1] = max(dp[v][1] + max(dp[u][0], dp[u][1]), dp[v][0] + dp[u][0] + 1);
dp[v][0] += max(dp[u][0], dp[u][1]);
}
}
}
int main() {
fast_io;
int n = 0, m;
cin >> m;
for (int i = 1; i <= m; i++) {
string s, t;
cin >> s >> t;
if (!mp[s])
mp[s] = ++n;
if (!mp[t])
mp[t] = ++n;
int v = mp[t], u = mp[s];
if (v == u) {
r.pb(u);
}
else {
adj[u].pb(v);
adj[v].pb(u);
}
}
if (n & 1) kill(-1);
int cnt = 0;
// for (auto u: r) cout << u << ' ';
for (auto u: r) {
dfs(u);
cnt += max(dp[u][0], dp[u][1]);
}
cout << n - cnt << '\n';
return 0;
}
