#include<bits/stdc++.h>
#define int long long
using namespace std;
const int maxn = 1e5 + 5;
int n, ans;
map<string,int> mp;
set<int> s;
queue<int> q;
int deg[maxn], nxt[maxn], nxt_two[maxn], mark[maxn];
signed main()
{
ios_base::sync_with_stdio(false);
cin.tie(0); cout.tie(0);
// freopen("test.inp", "r", stdin);
// freopen("test.out", "w", stdout);
cin >> n;
for (int i = 1; i <= n; i++)
{
string u, v;
cin >> u >> v;
if (!mp.count(u)) mp[u] = mp.size() + 1;
if (!mp.count(v)) mp[v] = mp.size() + 1;
nxt[mp[u]] = mp[v];
deg[mp[v]]++;
}
if (mp.size() & 1) return cout << -1, 0;
n = mp.size();
for (int i = 1; i <= n; i++)
nxt_two[i] = nxt[nxt[i]],
s.emplace(i),
mark[i] = 1;
int cnt = n;
for (int i = 1; i <= n; i++)
if (mark[i] && nxt_two[i] == i && nxt[i] != i)
{
mark[i] = mark[nxt[i]] = 0;
s.erase(i); s.erase(nxt[i]);
cnt -= 2;
}
for (int i = 1; i <= n; i++)
if (mark[i] && deg[i] == 0)
q.emplace(i);
while (cnt)
{
if (q.empty()) q.emplace(*s.begin());
int u = q.front();
q.pop();
mark[u] = 0;
s.erase(u);
ans++;
cnt--;
if (mark[nxt[u]])
{
s.erase(nxt[u]);
cnt--;
mark[nxt[u]] = 0;
deg[nxt_two[u]]--;
if (mark[nxt_two[u]] && deg[nxt_two[u]] == 0)
q.emplace(nxt_two[u]);
}
}
cout << ans;
}
