#include <bits/stdc++.h>
using namespace std;
#define int long long
#define all(x) x.begin(), x.end()
#define debug(x) cout << #x << " " << (x) << endl;
struct DSU {
vector<int> p, sz;
DSU(int n) {
p = vector<int>(n), sz = vector<int>(n, 1);
iota(all(p), 0ll);
}
int find(int a) {
if (a == p[a]) return p[a];
return p[a] = find(p[a]);
}
void join(int u, int v) {
int r1 = find(u);
int r2 = find(v);
if (r1 == r2) return;
p[r2] = r1;
sz[r1] += sz[r2];
}
};
void solve() {
int n;cin>>n;
vector<pair<string, string>> a(n);
set<string> st;
map<string, int> mp;
for (int i = 0; i < n; i++) {
cin >> a[i].first >> a[i].second;
st.insert(a[i].first);
st.insert(a[i].second);
}
int m = 0;
for (auto el : st) mp[el] = m++;
DSU dsu(m);
vector<int> nxt(m);
for (int i = 0; i < n; i++) {
dsu.join(mp[a[i].first], mp[a[i].second]);
nxt[mp[a[i].first]] = mp[a[i].second];
}
int ans = 0;
int leftover = 0;
for (int i = 0; i < m; i++) {
if (dsu.find(i) == i) {
// debug(dsu.sz[i]);
if (dsu.sz[i] & 1) leftover++;
ans += dsu.sz[i]/2;
}
}
if (leftover & 1) {
cout << -1 << endl;
return;
}
// debug(ans);
int x = 0;
for (int i = 0; i < m; i++) {
if (nxt[nxt[i]] == i && nxt[i] != i) x++;
}
cout << ans + leftover - x/2 << endl;
}
int32_t main() {
solve();
return 0;
}
