#include <bits/stdc++.h>
using namespace std;
const int nx=1e5+5;
int n, t, vs[nx], l[nx], res, cnt, lv[nx];
string su[nx], sv[nx];
map<string, int> mp;
int main()
{
cin.tie(NULL)->sync_with_stdio(false);
cin>>n;
for (int i=1; i<=n; i++) cin>>su[i]>>sv[i], mp[su[i]]=mp[sv[i]]=0;
if (n%2) return cout<<-1, 0;
for (auto &[x, y]:mp) y=++t;
for (int i=1; i<=n; i++)
{
l[mp[su[i]]]=mp[sv[i]];
if (mp[su[i]]!=mp[sv[i]]) lv[mp[sv[i]]]=1;
}
for (int i=1; i<=n; i++)
{
if (lv[i]) continue;
int sz=1, tmp=i;
while (l[tmp]!=tmp) sz++, tmp=l[tmp];
//cout<<"debug "<<i<<' '<<sz<<'\n';
res+=sz/2;
if (sz%2) res++;
}
cout<<res;
}
