#include <iostream>
#include <string>
#include <math.h>
#include <algorithm>
#include <vector>
#include <string.h>
#include <numeric>
#include <iostream>
#include <queue>
#include <assert.h>
#include <map>
#include <set>
#include <limits.h>
#include <random>
#include <chrono>
using namespace std;
#define ll long long
#define ld long double
const int MX = 200005;
const int BLOCK = 105;
const ll mod = 1e9 + 7;
const ll inv2 = (mod + 1) / 2;
const int dxh[] = {1, 1, -1, -1, 2, 2, -2, -2};
const int dyh[] = {2, -2, 2, -2, 1, -1, 1, -1}; // horse
const int dx[] = {1, 0, -1, 0};
const int dy[] = {0, 1, 0, -1}; // adj
const int dxd[] = {1, 1, 1, 0, -1, -1, -1, 0};
const int dyd[] = {1, 0, -1, -1, -1, 0, 1, 1}; // diag
int n, to[MX], indeg[MX], vis[MX], ans = 0;
vector<pair<int, int> > edge;
map<string, int> mp;
int mpid = 0;
int main(){
cin.tie(0) -> sync_with_stdio(0);
cin >> n;
if(n % 2){
cout << -1 << endl;
return 0;
}
string a, b;
for(int i = 0; i < n; i ++){
cin >> a >> b;
if(!mp.count(a)) mp[a] = mpid ++;
if(!mp.count(b)) mp[b] = mpid ++;
int u = mp[a], v = mp[b];
edge.push_back({u, v});
indeg[v] ++;
to[u] = v;
}
queue<int> lonely;
for(int i = 0; i < n; i ++){
if(!indeg[i]) lonely.push(i);
if(to[i] != i && to[to[i]] == i)
vis[i] = vis[to[i]] = 1;
}
for(; lonely.size();){
int nw = lonely.front();
lonely.pop();
vis[nw] = 1;
if(!vis[to[nw]]){
vis[to[nw]] = 1;
indeg[to[to[nw]]] --;
if(!indeg[to[to[nw]]])
lonely.push(to[to[nw]]);
}
ans ++;
}
for(int i = 0; i < n; i ++) if(!vis[i]){
vis[i] = 1;
int nw = i, cnt = 1;
for(; !vis[to[nw]]; nw = to[nw], cnt ++)
vis[to[nw]] = 1;
ans += (cnt + 1) / 2;
}
cout << ans << endl;
return 0;
}
