#include <bits/stdc++.h>
using namespace std;
const int N = 1e5 + 3;
const int inf32 = 1e9;
int n, to[N], in_deg[N];
namespace sub1 {
int dp[1 << 20];
void solve(){
fill(dp + 1, dp + (1 << n), inf32);
vector<int> gr;
for (int mask = 1; mask < (1 << n); ++mask) if (__builtin_popcount(mask) % 2 == 0){
gr.clear();
for (int i = 0; i < n; ++i) if (mask >> i & 1)
gr.push_back(i);
for (int i = 0; i < (int)gr.size(); ++i){
for (int j = i + 1; j < (int)gr.size(); ++j){
int u = gr[i], v = gr[j];
if (dp[mask ^ (1 << u) ^ (1 << v)] != inf32){
int c;
if (to[u] == v && to[v] == u) c = 0;
else if (to[u] == v || to[v] == u) c = 1;
else c = 2;
dp[mask] = min(dp[mask], dp[mask ^ (1 << u) ^ (1 << v)] + c);
}
}
}
}
cout << dp[(1 << n) - 1] << '\n';
}
};
namespace sub2 {
bool check(){
for (int i = 0; i < n; ++i){
if (in_deg[i] == 0) return false;
}
return true;
}
vector<bool> vis(N);
void solve(){
int res = 0;
for (int i = 0; i < n; ++i) if (!vis[i]){
int sz = 1;
vis[i] = true;
for (int u = to[i]; u != i; u = to[u]){
++sz;
vis[u] = true;
}
if (sz > 2) res += sz / 2;
if (sz & 1) ++res;
}
cout << res << '\n';
}
};
int main(){
ios_base::sync_with_stdio(false);
cin.tie(NULL);
cin >> n;
if (n & 1){
cout << "-1\n";
return 0;
}
vector<pair<string, string>> R;
vector<string> people;
for (int i = 0; i < n; ++i){
string a, b;
cin >> a >> b;
R.emplace_back(a, b);
people.push_back(a), people.push_back(b);
}
sort(begin(people), end(people));
people.erase(unique(begin(people), end(people)), end(people));
for (auto [a, b] : R){
int i = lower_bound(begin(people), end(people), a) - begin(people);
int j = lower_bound(begin(people), end(people), b) - begin(people);
to[i] = j;
++in_deg[j];
}
if (n <= 20){
sub1::solve();
return 0;
}
if (sub2::check()){
sub2::solve();
return 0;
}
}
