#include <bits/stdc++.h>
using namespace std;
const int nx=1e5+5;
int n, t, vs[nx], res, cnt, l[nx], c[nx], used[nx];
string su[nx], sv[nx];
map<string, int> mp;
vector<int> d[nx], cy[nx];
int dfs(int u, int rt)
{
vs[u]=1;
for (auto v:d[u]) if (v==rt||(!vs[v]&&dfs(v, rt))) return c[u]=cnt, cy[cnt].push_back(u), 1;
return 0;
}
void dfstree(int u, int p)
{
for (auto v:d[u]) if (c[v]==-1) dfstree(v, u);
if (u!=p&&!used[u]&&!used[p]) res++, used[u]=used[p]=1;
}
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]]=i, c[i]=-1;
if (n%2) return cout<<-1, 0;
for (int i=1; i<=n; i++)
{
l[mp[su[i]]]=mp[sv[i]];
if (mp[su[i]]!=mp[sv[i]]) d[mp[sv[i]]].push_back(mp[su[i]]);
else cy[cnt++].push_back(i);
}
for (int i=1; i<=n; i++) if (!vs[i]&&dfs(i, i)) cnt++;
for (int i=0; i<cnt; i++)
{
int cnt=0;
for (auto x:cy[i]) dfstree(x, x), cnt+=(!used[x]);
//for (auto x:cy[i]) cout<<"debug "<<x<<' '<<used[x]<<'\n';
//cout<<"count "<<cnt<<'\n';
if (cnt==cy[i].size())
{
if (cy[i].size()==2) res+=2;
else res+=cy[i].size()/2;
}
for (auto x:cy[i])
{
if (used[x]&&!used[l[x]])
{
int tmp=l[x], sz=1;
while (!used[l[tmp]]) sz++, tmp=l[tmp];
res+=sz/2;
}
}
}
cout<<n-res;
}
/*
8
a b
b c
c a
d a
e f
f e
g h
h g
10
a b
b b
c d
d e
e e
f g
g h
h h
x x
y y
8
a b
b c
c d
d a
e a
f b
g c
h d
12
a b
b c
c d
d a
e a
f b
g c
h d
x e
y f
m m
n n
*/
