#include <bits/stdc++.h>
using namespace std;
const int MN=1e5+2;
int N;
set<int> fromgr[MN];
int togr[MN];
bool vis[MN];
int dpst[MN], dpew[MN];
void dfs(int u, int root) {
vis[u] = 1;
if(fromgr[u].empty()) {
dpst[u] = 1;
dpew[u] = 0;
return;
}
dpst[u] = 1e9;
dpew[u] = 0;
for(int v : fromgr[u]) {
if(v == root) continue;
dfs(v, root);
dpew[u] += dpst[v];
}
for(int v : fromgr[u]) {
if(v == root) continue;
dpst[u] = min(dpst[u], dpew[u] - dpst[v] + dpew[v] + 1);
}
if(dpst[u] == (int)1e9) dpst[u] = 1;
}
int solve(int u) {
vector<bool> seen(N, 0);
// find root
seen[u] = 1;
while(!seen[togr[u]]) {
u = togr[u];
seen[u] = 1;
}
// relationship
if(togr[u] != u && togr[togr[u]] == u) {
dfs(u, togr[u]);
dfs(togr[u], u);
return dpew[u] + dpew[togr[u]];
}
dfs(u, u);
int ans = min(dpst[u], dpew[u] + 1);
fromgr[togr[togr[u]]].erase(togr[u]);
dfs(u, u);
return min(ans, dpew[u] + dpew[togr[u]] + 1);
}
int main() {
ios::sync_with_stdio(0); cin.tie(0);
cin>>N;
if(N%2) {
cout<<"-1\n";
return 0;
}
unordered_map<string, int> id;
vector<string> pts;
for(int i=0; i<N; ++i) {
string a,b;
cin>>a>>b;
id[a] = i;
pts.emplace_back(b);
}
for(int i=0; i<N; ++i) {
togr[i] = id[pts[i]];
fromgr[id[pts[i]]].emplace(i);
}
int ans = 0;
for(int i=0; i<N; ++i) {
if(vis[i]) continue;
ans += solve(i);
}
// for(int i=0; i<N; ++i) cout<<dpst[i]<<' '<<dpew[i]<<'\n';
cout<<ans<<'\n';
return 0;
}
