#include <bits/stdc++.h>
using namespace std;
#define fast ios::sync_with_stdio(0);cin.tie(0);
#define s second
#define f first
typedef long long ll;
const ll MOD = 998244353;
const ll LOGN = 21;
const ll MAXN = 2e5 + 100;
vector<vector<int>> graph;
map<string, int> mp;
int last = 0;
int main() {
fast
int n;
cin >> n;
graph = vector<vector<int>>(n+1, vector<int>());
for (int i = 0; i < n; i++) {
string x, y;
cin >> x >> y;
mp[x] = mp[x] ? mp[x] : last++;
mp[y] = mp[y] ? mp[y] : last++;
graph[mp[x]].push_back(mp[y]);
graph[mp[y]].push_back(mp[x]);
}
if (n % 2)
cout << "-1\n";
else
cout << n / 2 << "\n";
}
