#include <bits/stdc++.h>
using namespace std;
const int N = 101010;
int n, go[N], vis[N]; bool dead[N];
vector<int> g[N];
map<string, int> mp;
map<int, int> cant;
array<int, 3> dfs(int p, int v) {
int s = 0, t = 1, sz = 1;
for (int x : g[v]) {
if (p == x || dead[x])
continue;
if (cant[v] == x)
continue;
auto [y, z, w] = dfs(v, x);
s += max(y, z);
t += y;
sz += w;
}
return {s, t, sz};
}
int main() {
cin.tie(0); ios_base::sync_with_stdio(0);
cin >> n;
int sz = 0;
for (int i = 1; i <= n; i++) {
string s, t; cin >> s >> t;
int u = (mp[s] = (mp[s]) ? mp[s] : ++sz);
int v = (mp[t] = (mp[t]) ? mp[t] : ++sz);
go[u] = v;
g[u].push_back(v);
g[v].push_back(u);
}
if (n % 2 == 1)
return !(cout << -1);
int ans = 0;
vector<int> vtx;
for (int i = 1; i <= n; i++) {
if (vis[i] == 2)
continue;
int v = i;
while (!vis[v]) {
vtx.push_back(v);
vis[v] = 1;
v = go[v];
}
if (vis[v] == 1) {
// not using the edge
int s = 0;
cant[go[v]] = v;
cant[v] = go[v];
auto [x, y, z] = dfs(v, v);
s = z - max(x, y);
// using the edge
if (go[v] != v) {
int t = 1;
// cycle is length 2
if (v == go[go[v]])
t = 2;
dead[v] = 1;
dead[go[v]] = 1;
for (int x : g[v])
if (go[v] != x) {
auto [a, b, c] = dfs(v, x);
t += c - max(a, b);
}
for (int x : g[go[v]])
if (v != x) {
auto [a, b, c] = dfs(go[v], x);
t += c - max(a, b);
}
ans += max(s, t);
}
else
ans += s;
}
// initialize
for (int x : vtx)
vis[x] = 2;
vector<int>().swap(vtx);
}
cout << n - ans;
}
