#include <bits/stdc++.h>
using namespace std;
#define ll uint_fast64_t
const ll base = 1e6;
const int MAX = 100001;
int n,edges[MAX],value;
string s,t;
map <string,int> m;
map <ll,bool> if_match;
bool if_deleted[MAX],if_paired[MAX];
string graph[MAX];
queue <int> q;
int dfs(int a,int c,int length)
{
if_deleted[a] = true;
if (c == m[graph[a]])return length + 1;
if(if_deleted[m[graph[a]]]||if_paired[m[graph[a]]])return length + 1;
return dfs(m[graph[a]], c, length + 1);
return length + 1;
}
int main()
{
ios_base::sync_with_stdio(false);
cin.tie(NULL);
cin>>n;
if(n%2)
{
printf("-1");
return 0;
}
for(int i = 1;i<=n;++i)
{
cin>>s>>t;
m[s] = i;
graph[m[s]] = t;
}
for(int i = 1;i<=n;++i) {
if(if_match[ll(m[graph[i]]) * base + ll(i)])
{
if_deleted[i] = true;
if_deleted[m[graph[i]]] = true;
continue;
}
if_match[ll(i) * base + ll(m[graph[i]])] = true;
}
for(int i = 1;i<=n;++i)
{
if(if_deleted[i])continue;
if(!if_deleted[m[graph[i]]])edges[m[graph[i]]]++;
}
for(int i = 1;i<=n;++i)if(!edges[i]&&!if_deleted[i])q.push(i);
while(!q.empty())
{
int p = q.front();
q.pop();
if(if_paired[p]||if_deleted[p])continue;
if(!if_paired[m[graph[p]]])
{
if_paired[p] = true;
if_paired[m[graph[p]]] = true;
++value;
--edges[m[graph[m[graph[p]]]]];
if(!edges[m[graph[m[graph[p]]]]]) q.push(m[graph[m[graph[p]]]]);
}
}
//cout<<value<<endl;
for(int i = 1 ;i<=n;++i)
{
if(!if_deleted[i]&&!if_paired[i])
{
int h = dfs(i,i,0);
// cout<<h<<endl;
if(h%2)value+= (h>>1) +1;
else value += (h>>1);
}
}
cout<<value<<endl;
}
