#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define rep(i, n) for (int i = 0; i < (n); i++)
#define st first
#define nd second
#define pb push_back
map<string, int> m;
int main() {
ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
int n;
cin >> n;
if (n % 2 == 1) {
cout << -1 << '\n';
return 0;
}
vector<pair<string, string>> vec;
set<string> S;
rep(i, n) {
string a, b;
cin >> a >> b;
vec.pb({a, b});
S.insert(a);
S.insert(b);
}
int kt = 1;
for (auto s: S) {
m[s] = kt;
kt++;
}
int dokad[n + 1];
dokad[0] = 0;
rep(i, n) {
dokad[m[vec[i].st]] = m[vec[i].nd];
}
bool uzyte[n + 1];
rep(i, n + 1) {
uzyte[i] = false;
}
for (int v = 1; v <= n; v++) {
int w = dokad[v];
if (uzyte[w]) {
dokad[v] = v;
continue;
}
if (w != v && dokad[w] == v) {
uzyte[v] = true;
uzyte[w] = true;
}
}
int ans = 0;
int ile[n + 1];
rep(i, n + 1) {
ile[i] = 0;
}
for (int v = 1; v <= n; v++) {
int w = dokad[v];
if (uzyte[w]) {
dokad[v] = v;
}
if (dokad[v] != v) {
ile[dokad[v]]++;
}
}
queue<int> q;
for (int v = 1; v <= n; v++) {
if (uzyte[v]) continue;
if (ile[v] == 0) {
q.push(v);
}
}
while (!q.empty()) {
int v = q.front();
q.pop();
if (uzyte[v]) continue;
int w = dokad[v];
if (uzyte[w] || w == v) {
ans++;
uzyte[v] = true;
}
else {
uzyte[v] = true;
uzyte[w] = true;
ans++;
int z = dokad[w];
ile[z]--;
if (!uzyte[z] && ile[z] == 0) {
q.push(z);
}
}
}
for (int v = 1; v <= n; v++) {
if (uzyte[v]) continue;
int w = v;
int s = 0;
while (!uzyte[w]) {
uzyte[w] = true;
w = dokad[w];
s++;
}
ans += (s/2);
ans += (s % 2);
}
cout << ans << '\n';
return 0;
}
