#include <bits/stdc++.h>
#define ll long long
#define pii pair<int, int>
#define pll pair<ll, ll>
#define F first
#define S second
using namespace std;
int N, idx, pre[100005], vis[100005], visr[100005], deg[100005], ans;
bool used[100005];
map<string, int> mp;
vector<int> E[100005], rE[100005], newE[100005];
vector<pii> oE;
void dfs(int k, int r)
{
for (int i : E[k])
if (!vis[i] || (visr[i] == r && vis[i] == 1))
{
vis[i]++;
visr[i] = r;
dfs(i, r);
}
}
void tree_matching(int k)
{
for (int i : rE[k])
if (vis[i] != 2)
tree_matching(i);
for (int i : rE[k])
if (vis[i] != 2 && !used[i])
{
used[k] = used[i] = 1;
ans++;
break;
}
}
#define debug(arr) \
for (int i = 1; i <= N; i++) \
cout << arr[i] << " \n"[i == N];
signed main()
{
cin >> N;
if (N % 2 == 1)
{
cout << -1 << '\n';
return 0;
}
for (int i = 1; i <= N; i++)
{
string s, t;
cin >> s >> t;
if (mp[s] == 0)
mp[s] = ++idx;
if (mp[t] == 0)
mp[t] = ++idx;
oE.emplace_back(make_pair(mp[s], mp[t]));
}
for (pii p : oE)
if (!used[p.F])
{
int u = p.F, v = (used[p.S] ? p.F : p.S);
E[u].emplace_back(v);
rE[v].emplace_back(u);
}
mp.clear();
for (int i = 1; i <= N; i++)
if (!vis[i])
{
vis[i] = 1;
visr[i] = i;
dfs(i, i);
}
//cout << "vis ";
//debug(vis);
for (int i = 1; i <= N; i++)
if (vis[i] == 2)
tree_matching(i);
for (int i = 1; i <= N; i++)
if (!used[i])
ans++;
cout << ans << '\n';
}
