#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
#define ll long long
#define ld long double
#define ull unsigned long long
#define ff first
#define ss second
#define pii pair<int,int>
#define pll pair<long long, long long>
#define vi vector<int>
#define vl vector<long long>
#define pb push_back
#define rep(i, b) for(int i = 0; i < (b); ++i)
#define rep2(i,a,b) for(int i = a; i <= (b); ++i)
#define rep3(i,a,b,c) for(int i = a; i <= (b); i+=c)
#define count_bits(x) __builtin_popcountll((x))
#define all(x) (x).begin(),(x).end()
#define siz(x) (int)(x).size()
#define forall(it,x) for(auto& it:(x))
using namespace __gnu_pbds;
using namespace std;
typedef tree<int, null_type, less<int>, rb_tree_tag,tree_order_statistics_node_update> ordered_set;
//mt19937 mt;void random_start(){mt.seed(chrono::time_point_cast<chrono::milliseconds>(chrono::high_resolution_clock::now()).time_since_epoch().count());}
//ll los(ll a, ll b) {return a + (mt() % (b-a+1));}
const int INF = 1e9+50;
const ll INF_L = 1e18+40;
const ll MOD = 1e9+7;
int nxt_[100001];
vi sons[100001];
int color[100001];
bool del[100001];
vi path;
vi cycle;
void dfs(int v)
{
color[v] = 1;
path.pb(v);
if(color[nxt_[v]] == 0) dfs(nxt_[v]);
else
{
if(color[nxt_[v]] == 1)
{
while(path.back() != nxt_[v])
{
cycle.pb(path.back());
path.pop_back();
}
cycle.pb(nxt_[v]);
rep(i,siz(cycle)-1)
{
path.pb(1);
}
}
}
color[v] = 2;
path.pop_back();
}
pii dfs_dp(int v)
{
vector<pii> dp_sons;
forall(it,sons[v])
{
if(!del[it])
{
dp_sons.pb(dfs_dp(it));
}
}
int dp0 = 1;
int dp1 = 0;
int min_del = 0;
forall(it,dp_sons)
{
dp1 += it.ff;
dp0 += it.ff;
min_del = min(min_del,-it.ff+it.ss);
}
dp0 += min_del;
return {dp0,dp1};
}
int main()
{
ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
//random_start();
int n;
cin >> n;
vector<pair<string,string>> pairs;
map<string,int> m;
rep(i,n)
{
string s1,s2;
cin >> s1 >> s2;
m[s1] = 1;
m[s2] = 1;
pairs.pb({s1,s2});
}
if(n % 2 == 1)
{
cout << "-1\n";
return 0;
}
int cur = 0;
forall(it,m)
{
m[it.ff] = cur++;
}
rep(i,n)
{
nxt_[m[pairs[i].ff]] = m[pairs[i].ss];
sons[m[pairs[i].ss]].pb(m[pairs[i].ff]);
}
vector<pii> cycle_pairs;
vi trees;
int ans = 0;
rep(i,n)
{
if(color[i] == 0)
{
cycle = {};
dfs(i);
if(siz(cycle) != 0)
{
if(siz(cycle) == 1)
{
ans++;
forall(it,sons[cycle[0]])
{
if(it != cycle[0])
{
trees.pb(it);
}
}
}
else if(siz(cycle) == 2)
{
forall(it,sons[cycle[0]])
{
if(it != cycle[1])
{
trees.pb(it);
}
}
forall(it,sons[cycle[1]])
{
if(it != cycle[0])
{
trees.pb(it);
}
}
}
else
{
cycle_pairs.pb({cycle[0],cycle[1]});
}
}
}
}
forall(it,trees)
{
ans += dfs_dp(it).ff;
}
forall(it,cycle_pairs)
{
del[it.ff] = 1;
int ans1 = dfs_dp(it.ff).ff;
del[it.ff] = 0;
del[it.ss] = 1;
int ans2 = dfs_dp(it.ss).ff;
del[it.ss] = 0;
ans += min({ans1,ans2});
}
cout << ans << "\n";
}
