#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define sz(v) (int)(v).size()
int main() {
ios::sync_with_stdio(false); cin.tie(nullptr);
int n;
cin >> n;
if (n & 1) return cout << -1, 0;
vector<int> pr(n);
map<string, int> m;
for (int i = 0; i < n; i++) {
string s, t;
cin >> s >> t;
if (m.find(s) == m.end()) m[s] = (int)m.size();
if (m.find(t) == m.end()) m[t] = (int)m.size();
pr[m[s]] = m[t];
}
int cnt = 0;
vector<bool> vis(n);
for (int i = 0; i < n; i++) {
if (vis[i]) continue;
int x = i, cnt1 = 0;
while (!vis[x]) {
cnt1++;
vis[x] = true;
x = pr[x];
}
assert(x == i);
cnt += (cnt1 & 1);
}
assert(!(cnt & 1));
cout << (n >> 1) + (cnt >> 1);
}
