#include <bits/stdc++.h>
using namespace std;
const int maxn = 1e5+10;
int f[maxn];
int nivel[maxn], cc;
int ini[maxn], fim[maxn];
int mark[maxn];
bool done[maxn], incycle[maxn];
void dfs(int u)
{
mark[u] = 1;
if (mark[f[u]] == 1)
{
++cc;
mark[u] = 2;
ini[cc] = f[u], fim[cc] = u;
return;
}
if (!mark[f[u]]) dfs(f[u]);
nivel[u] = nivel[f[u]]+1;
mark[u] = 2;
}
bool comp(int a, int b)
{
if (incycle[a] && !incycle[b]) return 0;
if (!incycle[a] && incycle[b]) return 1;
return nivel[a] > nivel[b];
}
int main(void)
{
int n;
cin >> n;
map<string, int> mp;
int ind = 0;
for (int i = 1; i <= n; i++)
{
string s1, s2;
cin >> s1 >> s2;
if (mp.find(s1) == mp.end()) mp[s1] = ++ind;
if (mp.find(s2) == mp.end()) mp[s2] = ++ind;
f[mp[s1]] = mp[s2];
}
if (n&1)
{
cout << "-1\n";
return 0;
}
for (int i = 1; i <= n; i++)
if (!mark[i])
dfs(i);
int ans = n;
vector<int> v;
for (int i = 1; i <= n; i++)
v.push_back(i);
for (int i = 1; i <= cc; i++)
{
for (int u = ini[i]; ; u = f[u])
{
incycle[u] = 1;
if (u == fim[i]) break;
}
}
sort(v.begin(), v.end(), comp);
for (int i = 1; i <= n; i++)
{
if (done[i]) continue;
if (f[i] != i && f[f[i]] == i)
{
done[i] = done[f[i]] = 1;
ans -= 2;
}
}
for (auto u: v)
{
if (done[u]) continue;
if (f[u] != u && !done[f[u]])
{
ans--;
done[u] = done[f[u]] = 1;
}
}
cout << ans << "\n";
}
