#include<bits/stdc++.h>
#include<ext/pb_ds/assoc_container.hpp>
#include<ext/pb_ds/tree_policy.hpp>
using namespace std;
using namespace __gnu_pbds;
typedef long long ll;
typedef long double ld;
#define pb push_back
#define pii pair<int, int>
#define pll pair<ll, ll>
#define st first
#define nd second
#define ordered_set tree<int, null_type, less<int>, rb_tree_tag, tree_order_statistics_node_update>
#define debug false
const int MAXN = 100 * 1000 + 17;
map<string, int> m;
vector<int> graf[MAXN];
bool uzyte[MAXN];
deque<int> d;
int deg[MAXN];
int main () {
ios::sync_with_stdio(0); cin.tie(0); cout.tie(0);
int n;
cin >> n;
string s, t;
int cnt = 0;
for (int i = 0; i < n; i ++) {
cin >> s >> t;
if (m[s] == 0) {
cnt ++;
m[s] = cnt;
}
if (m[t] == 0) {
cnt ++;
m[t] = cnt;
}
graf[m[s]].pb(m[t]);
deg[m[t]] ++;
}
if (n % 2 == 1) {
cout << -1 << "\n";
return 0;
}
for (int i = 1; i <= n; i ++) {
if (i == graf[i][0]) {
continue;
}
if (i == graf[graf[i][0]][0]) {
uzyte[graf[i][0]] = true;
uzyte[i] = true;
}
}
int wyn = 0;
for (int i = 1; i <= n; i ++) {
if (deg[i] == 0) {
d.pb(i);
}
}
while (!d.empty()) {
int v = d.front();
d.pop_front();
int o = graf[v][0];
if (uzyte[v] || uzyte[o]) {
continue;
}
wyn ++;
uzyte[v] = true;
uzyte[o] = true;
for (auto x : graf[o]) {
deg[x] --;
if (deg[x] == 0) {
d.pb(x);
}
}
}
for (int i = 1; i <= n; i ++) {
if (!uzyte[i]) {
cnt = 1;
int v = i;
uzyte[i] = 1;
while (!uzyte[graf[v][0]]) {
v = graf[v][0];
uzyte[v] = 1;
cnt ++;
}
//cout << cnt << "\n";
wyn += (cnt/2 + cnt % 2);
}
}
cout << wyn << "\n";
return 0;
}
