#include<bits/stdc++.h>
#define newl '\n'
const int N = 1e5 + 10;
const int V = 1e7 + 10;
const long long INF = 1e18;
const long long M = 1e9 + 7;
struct Subtask{
int n,id;
std::vector<int> par,f;
std::map<std::string,int> m;
int get(std::string str){
if(m.find(str) == m.end()){
m[str] = id++;
}
return m[str];
}
Subtask() : id(0){
}
void readData(){
std::cin >> n;
par.assign(n + 1,0);
f.assign(1 << n,V);
for(int i = 1;i <= n;++i){
std::string su,sv;
std::cin >> su >> sv;
int u = get(su);
int v = get(sv);
par[u] = v;
}
}
void solve(){
f[0] = 0;
for(int mask = 0;mask < (1 << n);++mask){
std::vector<int> vec;
for(int i = 0;i < n;++i){
if(!((mask >> i) & 1)){
vec.emplace_back(i);
}
}
for(int i = 0;i < (int)vec.size();++i){
for(int j = i + 1;j < (int)vec.size();++j){
int u = vec[i];
int v = vec[j];
// std::cout << "IMP " << mask << " " << u << " " << v << newl;
int nm = (mask | (1 << u) | (1 << v));
f[nm] = std::min(f[nm],f[mask] + (par[u] != v) + (par[v] != u));
}
}
}
// assert(par[6] == 7);
// assert(par[7] == 6);
// std::cout << f[(1 << 6) | (1 << 7)] << newl;
std::cout << (f[(1 << n) - 1] == V ? -1 : f[(1 << n) - 1]);
}
void main(){
readData();
solve();
}
};
struct Solver{
int n,id,ans = 0;
std::vector<bool> match;
std::vector<std::vector<int>> adj,adj_rev;
std::map<std::string,int> m;
std::vector<int> par,vis,deg;
int get(std::string str){
if(m[str] == 0){
m[str] = ++id;
}
return m[str];
}
Solver() : id(0){
}
void readData(){
std::cin >> n;
match.assign(n + 1,0);
adj.resize(n + 1);
adj_rev.resize(n + 1);
par.assign(n + 1,0);
vis.assign(n + 1,0);
deg.assign(n + 1,0);
for(int i = 1;i <= n;++i){
std::string su,sv;
std::cin >> su >> sv;
int u = get(su);
int v = get(sv);
par[u] = v;
adj[par[u]].emplace_back(u);
adj_rev[u].emplace_back(par[u]);
++deg[par[u]];
// std::cout << u << " " << v << newl;
}
}
void prepare(){
std::queue<int> q;
for(int i = 1;i <= n;++i){
if(deg[i] == 0){
q.push(i);
}
}
while(!q.empty()){
int u = q.front();
q.pop();
for(int v : adj_rev[u]){
--deg[v];
if(deg[v] == 0){
q.push(v);
}
}
}
}
void dfs(int u){
// if(u == 3){
// std::cerr
// }
vis[u] = true;
for(int v : adj[u]){
if(vis[v] || v == u){
continue;
}
dfs(v);
if(!match[v] && !match[u]){
match[u] = match[v] = true;
++ans;
}
}
}
void dfs1(int u){
// assert(deg[u] > 0);
vis[u] = true;
for(int v : adj[u]){
if(vis[v] || v == u || deg[v] > 0){
continue;
}
// assert(deg[v] == 0);
dfs1(v);
if(!match[v] && !match[u]){
match[u] = match[v] = true;
++ans;
}
}
}
void dfs_cycle(int u){
// std::cerr << "IMP " << u << newl;
vis[u] = true;
for(int v : adj[u]){
if(deg[v] == 0){
continue;
}
if(!vis[v]){
dfs_cycle(v);
}
if(!match[v] && !match[u]){
match[u] = match[v] = true;
++ans;
}
}
}
void solve(){
// for(int i = 1;i <= n;++i){
// std::cout << deg[i] << " ";
// }
// std::cout << newl;
// assert(par[3] != 1);
// for(int v : adj[1]){
// std::cout << "NEXT 1 " << v << newl;
// }
if(n & 1){
std::cout << -1;
return;
}
for(int i = 1;i <= n;++i){
if(par[par[i]] == i){
if(par[i] > i){
match[i] = match[par[i]] = true;
vis[i] = vis[par[i]] = true;
}
dfs(i);
dfs(par[i]);
}else{
if(par[i] != i && deg[i] > 0){
// assert(false);
dfs1(i);
}
}
}
// for(int i = 1;i <= n;++i){
// std::cout << match[i] << " ";
// }
// std::cout << newl;
// std::cout << "IMP " << ans << newl;
// assert(match[par[3]] == true && !vis[3]);
for(int i = 1;i <= n;++i){
if(par[i] == i){
dfs(i);
}
}
std::fill(vis.begin() + 1,vis.end(),0);
for(int i = 1;i <= n;++i){
if(deg[i] > 0 && !vis[i]){
dfs_cycle(i);
}
}
for(int i = 1;i <= n;++i){
// std::cout << match[i] << " ";
ans += !match[i];
}
std::cout << ans;
}
void main(){
readData();
prepare();
solve();
}
};
int main(){
std::ios_base::sync_with_stdio(false);
std::cin.tie(nullptr);std::cout.tie(nullptr);
// freopen("A.inp","r",stdin);
// freopen("A.out","w",stdout);
Solver().main();
return 0;
}
