#include <bits/stdc++.h>
using namespace std;
#define arr array
#define vct vector
#define hmap unordered_map
#define str string
#define pii pair<int, int>
#define hset unordered_set
const int MX_N = 1e5 + 5;
int n;
arr<int, MX_N> ot;
arr<int, MX_N> in_cnt;
arr<bool, MX_N> dltd;
set<pii> ord; // {in_cnt[u], u}
void intls() {
for (int u = 1; u <= n; u++) in_cnt[ot[u]]++;
for (int u = 1; u <= n; u++) ord.insert({in_cnt[u], u});
}
void dlt(int u) {
dltd[u] = true;
ord.erase({in_cnt[u], u});
if (dltd[ot[u]]) return;
ord.erase({in_cnt[ot[u]], ot[u]});
in_cnt[ot[u]]--;
ord.insert({in_cnt[ot[u]], ot[u]});
}
int prcmp() {
int ans = 0;
for (int u = 1; u <= n; u++) {
if (dltd[u]) continue;
if (ot[u] == u) continue;
if (ot[ot[u]] != u) continue;
ans += 2;
dlt(u), dlt(ot[u]);
}
return ans;
}
int grdy() {
int ans = 0;
while (ord.size()) {
int u = ord.begin()->second;
if (ot[u] == u || dltd[ot[u]]) { dlt(u); continue; }
ans++;
dlt(u), dlt(ot[u]);
}
return ans;
}
int main() {
// freopen("lv.in", "r", stdin);
cin >> n;
hmap<int, str> ot_str;
hmap<str, int> ind;
for (int u = 1; u <= n; u++) {
str x, y; cin >> x >> y;
ind[x] = u, ot_str[u] = y;
}
for (int u = 1; u <= n; u++) ot[u] = ind[ot_str[u]];
if (n % 2 == 1) { cout << -1 << endl; exit(0); }
intls();
int ans = prcmp() + grdy();
cout << n - ans << endl;
}
