#include <bits/stdc++.h>
#define int long long
#define fastio ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
using namespace std;
int idx;
map<string,int> m;
int assign(string x){
if(m.find(x)==m.end()){
m[x] = idx++;
}
return m[x];
}
signed main(){
fastio
int n;
cin >> n;
int arr[n];
for(int i = 0;i < n;i++){
string a,b;
cin >> a >> b;
int x = assign(a);
int y = assign(b);
arr[x] = y;
}
if(n%2){
cout << -1 << "\n";
return 0;
}
int dp[1<<n];
fill(dp,dp+(1<<n),1e18);
dp[0] = 0;
for(int mask = 0;mask < (1<<n); mask++){
for(int i = 0;i < n;i++){
if(mask&(1<<i)) continue;
for(int j = 0;j < n;j++){
if(i==j||mask&(1<<j)) continue;
dp[mask|(1<<i)|(1<<j)] = min(dp[mask]+(arr[i]!=j)+(arr[j]!=i),dp[mask|(1<<i)|(1<<j)]);
}
}
}
cout << dp[(1<<n)-1] << "\n";
}
