#include<bits/stdc++.h>
#define debug(x) cerr << #x << " " << x << "\n"
#define debugs(x) cerr << #x << " " << x << " "
#pragma GCC optimize("Ofast")
#define int long long
using namespace std;
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
int range_rng(int l, int r)
{
return uniform_int_distribution<int>(l,r)(rng);
}
map<string,int> mp;
map<int,bool> in_cycle;
int nxt[100005];
bool visited[100005];
vector<int> curr_cycle;
vector<int> in_edge[100005];
int dp[100005][2];
int dp2[100005][2];
int cnt;
int ans;
bool ret;
int gettem;
int actual_sz;
int remain;
void dfs(int node)
{
if (visited[node])
{
ret=true;
gettem=node;
return;
}
visited[node]=1;
dfs(nxt[node]);
if (ret==true)
{
curr_cycle.push_back(node);
in_cycle[node]=1;
if (gettem==node)
ret=false;
}
}
void dfs2(int node)
{
visited[node]=1;
actual_sz++;
int s,mx;
s=0;
for (auto x : in_edge[node])
{
dfs2(x);
s+=max(dp[x][1],dp[x][0]);
}
dp[node][0]=s;
mx=0;
for (auto x : in_edge[node])
mx=max(mx,s-max(dp[x][1],dp[x][0])+dp[x][0]+1);
dp[node][1]=mx;
}
void compute_dp(int root)
{
int s,mx;
s=0;
for (auto x : in_edge[root])
{
if (in_cycle.find(x)==in_cycle.end())
{
dfs2(x);
s+=max(dp[x][1],dp[x][0]);
}
}
dp[root][0]=s;
mx=0;
for (auto x : in_edge[root])
{
if (in_cycle.find(x)==in_cycle.end())
mx=max(mx,s-max(dp[x][1],dp[x][0])+dp[x][0]+1);
}
dp[root][1]=mx;
}
void solve()
{
actual_sz=curr_cycle.size();
for (auto x : curr_cycle)
{
compute_dp(x);
}
if (curr_cycle.size()==2)
{
remain-=2;
ans+=dp[curr_cycle[0]][0]+dp[curr_cycle[1]][0];
return;
}
if (curr_cycle.size()==1)
{
ans+=max(dp[curr_cycle[0]][0],dp[curr_cycle[0]][1]);
return;
}
int mx,i;
mx=0;
dp2[0][1]=max(dp[curr_cycle[0]][1],dp[curr_cycle[0]][0]);
dp2[0][0]=dp[curr_cycle[0]][0];
for (i=1; i<curr_cycle.size(); i++)
{
dp2[i][1]=max(dp2[i-1][0]+dp[curr_cycle[i]][0]+1,max(dp2[i-1][0],dp2[i-1][1])+max(dp[curr_cycle[i]][1],dp[curr_cycle[i]][0]));
dp2[i][0]=max(dp2[i-1][1],dp2[i-1][0])+dp[curr_cycle[i]][0];
}
mx=max(mx,max(dp2[curr_cycle.size()-1][1],dp2[curr_cycle.size()-1][0]));
///avem un caz dubios : cuplam 1 cu n => hai sa il tratam separat
dp2[0][1]=dp[curr_cycle[0]][0]+dp[curr_cycle[curr_cycle.size()-1]][0]+1;
dp2[0][0]=dp[curr_cycle[0]][0];
for (i=1; i<curr_cycle.size()-1; i++)
{
dp2[i][1]=max(dp2[i-1][0]+dp[curr_cycle[i]][0]+1,max(dp2[i-1][0],dp2[i-1][1])+max(dp[curr_cycle[i]][1],dp[curr_cycle[i]][0]));
dp2[i][0]=max(dp2[i-1][1],dp2[i-1][0])+dp[curr_cycle[i]][0];
}
mx=max(mx,max(dp2[curr_cycle.size()-2][1],dp2[curr_cycle.size()-2][0]));
ans+=mx;
}
signed main()
{
/*
ifstream fin("memes.in");
ofstream fout("memes.out");
*/
ios_base::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
int n,i,j,casafiefrumos;
string s1,s2;
cin >> n;
cnt=0;
for (i=1; i<=n; i++)
{
cin >> s1 >> s2;
if (mp[s1]==0)
mp[s1]=++cnt;
if (mp[s2]==0)
mp[s2]=++cnt;
nxt[mp[s1]]=mp[s2];
in_edge[mp[s2]].push_back(mp[s1]);
}
if (n%2==1)
{
cout << "-1\n";
return 0;
}
ans=0;
remain=n;
for (i=1; i<=n; i++)
{
if (!visited[i])
{
curr_cycle.clear();
in_cycle.clear();
ret=false;
dfs(i);
solve();
}
}
remain-=ans*2;
casafiefrumos=ans+remain;
cout << casafiefrumos;
return 0;
}
