#include <bits/stdc++.h>
using namespace std;
#define MAXN 100001
#define int long long
int n;
vector<pair<string,string>> edges;
vector<string> kompresija;
vector<int> adj[2*MAXN];
bool pos[2*MAXN];
int idx[2*MAXN];
vector<int> toposort;
vector<vector<int>> cycles;
vector<int> graph[2*MAXN];
int dp[3][2*MAXN];
void dfs(int node)
{
pos[node]=true;
for (int sled:adj[node])
{
if (!pos[sled]) dfs(sled);
}
toposort.push_back(node);
}
void calculate(int node,int pret)
{
for (int sled:graph[node])
{
if (sled!=pret) calculate(sled,node);
}
int maxrazlika=-INT_MAX;
for (int sled:graph[node])
{
if (sled!=pret)
{
dp[0][node]+=max(dp[0][sled],dp[2][sled]);
dp[1][node]+=max(dp[0][sled],dp[2][sled]);
dp[2][node]+=max(dp[0][sled],dp[2][sled]);
maxrazlika=max(maxrazlika,dp[1][sled]-max(dp[0][sled],dp[2][sled]));
}
}
if (maxrazlika!=-INT_MAX) dp[2][node]+=maxrazlika+1;
}
int32_t main()
{
cin>>n;for (int i=1;i<=n;i++) {string s1,s2;cin>>s1>>s2;edges.push_back({s1,s2});kompresija.push_back(s1);kompresija.push_back(s2);}
if (n%2==1) {cout<<-1<<endl;return 0;}
sort(kompresija.begin(),kompresija.end());kompresija.erase(unique(kompresija.begin(),kompresija.end()),kompresija.end());
for (pair<string,string> edge:edges)
{
int u=lower_bound(kompresija.begin(),kompresija.end(),edge.first)-kompresija.begin()+1;
int v=lower_bound(kompresija.begin(),kompresija.end(),edge.second)-kompresija.begin()+1;
adj[u].push_back(v);
}
for (int node=1;node<=(int)kompresija.size();node++)
{
if (!pos[node]) dfs(node);
}
reverse(toposort.begin(),toposort.end());for (int i=0;i<(int)toposort.size();i++) idx[toposort[i]]=i;
for (int node=1;node<=(int)kompresija.size();node++) pos[node]=false;
for (int node=1;node<=(int)kompresija.size();node++)
{
for (int sled:adj[node])
{
if (idx[node]<idx[sled]) continue;
vector<int> cycle;cycle.push_back(node);int current=sled;pos[node]=true;
while (current!=node) {cycle.push_back(current);pos[current]=true;current=adj[current][0];}
cycles.push_back(cycle);
}
}
for (int node=1;node<=(int)kompresija.size();node++)
{
for (int sled:adj[node])
{
if (pos[node] and pos[sled]) continue;
graph[node].push_back(sled);graph[sled].push_back(node);
}
}
int answer=0;
for (vector<int> cycle:cycles)
{
for (int pos=1;pos<(int)cycle.size();pos++)
{
graph[cycle[pos]].push_back(cycle[pos-1]);
graph[cycle[pos-1]].push_back(cycle[pos]);
}
calculate(cycle[0],0);answer+=max(dp[0][cycle[0]],dp[2][cycle[0]]);
}
cout<<answer<<endl;
}
