#include <bits/stdc++.h>
#pragma GCC optimize("O3,unroll-loops")
#pragma GCC target("avx2,bmi,bmi2,popcnt,lzcnt,tune=native")
#define fio ios_base::sync_with_stdio(0);cin.tie(0);
#define ll long long
#define ld long double
#define en exit(0);
#define pb push_back
#define fi first
#define se second
using namespace std;
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
const int N = 1e5 + 5;
set<int> g[N];
int to[N], vis[N];
int dp[N][2];
pair<int,int> vert;
void dfs(int u)
{
vis[u] = 1;
int nxt = to[u];
if(!vis[nxt])
dfs(nxt);
else if(vis[nxt] == 1)
vert = {u, nxt};
vis[u] = 2;
}
void dfs2(int u)
{
dp[u][0] = dp[u][1] = 0;
int dif = 0;
for(auto v:g[u])
{
dfs2(v);
dp[u][0] += dp[v][1];
dp[u][1] += dp[v][1];
dif = max(dif, dp[v][1] - dp[v][0]);
}
dp[u][1] += 1 - dif;
dp[u][1] = min(dp[u][1], dp[u][0] + 1);
}
int try_from(int r)
{
g[to[r]].erase(r);
dfs2(r);
g[to[r]].insert(r);
return dp[r][1];
}
int main()
{
fio
// ifstream cin("in.in");
int n;
cin >> n;
if(n&1)
return cout << -1, 0;
map<string, int> mp;
int ind = 0;
for(int i = 1;i <= n;i++)
{
string a, b;
cin >> a >> b;
if(!mp.count(a))
mp[a] = ++ind;
if(!mp.count(b))
mp[b] = ++ind;
int u = mp[a], v = mp[b];
to[u] = v;
g[v].insert(u);
}
int res = 0;
for(int i = 1;i <= n;i++)
{
if(!vis[i])
{
vert = {-1, -1};
dfs(i);
if(vert.fi != -1)
{
if(to[vert.fi] == vert.se && to[vert.se] == vert.fi && vert.fi != vert.se)
{
int cnt = 0;
for(auto v:g[vert.fi])
if(v != vert.se)
cnt += try_from(v);
for(auto v:g[vert.se])
if(v != vert.fi)
cnt += try_from(v);
res += cnt;
}
else
{
int ans1 = try_from(vert.fi), ans2 = try_from(vert.se);
res += min(ans1, ans2);
}
}
}
}
cout << res;
return 0;
}
