#include <bits/stdc++.h>
using namespace std;
#define in ({int x=0;int c=getchar(),n=0;for(;!isdigit(c);c=getchar()) n=(c=='-');for(;isdigit(c);c=getchar()) x=x*10+c-'0';n?-x:x;})
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
int rnd(int l,int r)
{
return l+rng()%(r-l+1);
}
#define fasty ios_base::sync_with_stdio(0),cin.tie(0),cout.tie(0);
#define forinc(x,a,b) for(int x=a;x<=b;x++)
#define fordec(x,a,b) for(int x=a;x>=b;x--)
#define forv(a,b) for(auto&a:b)
#define fi first
#define se second
#define pb push_back
#define ll long long
#define mt make_tuple
#define all(a) a.begin(),a.end()
#define reset(f,x) memset(f,x,sizeof(f))
#define getbit(x,i) ((x>>(i))&1)
#define batbit(x,i) (x|(1ll<<(i)))
#define tatbit(x,i) (x&~(1<<(i)))
#define gg exit(0);
#define int long long
#define ii pair<int,int>
const int N = 1e5 + 100;
int n;
int cnt[22];
ii edge[N];
main()
{
fasty;
cin >> n;
forinc(i,1,n)
{
int u,v;
cin >> u >> v;
edge[i] = {u,v};
}
if(n<=20)
{
int res=1e8;
int lim=(1<<n)-1;
forinc(mask , 0 , lim)
{
memset(cnt,0,sizeof cnt);
forinc(i,1,n)
{
int u = edge[i].fi;
int v = edge[i].se;
if(getbit(mask , u-1)) cnt[v]++;
if(getbit(mask,v-1)) cnt[u]++;
}
int ok=1;
forinc(i,1,n)
{
if(cnt[i] != ok)
{
ok=0;
break;
}
}
int sl = __builtin_popcount(mask);
if(ok) res = min(res , sl);
}
if(res>=1e8)
{
cout << -1;
return 0;
}
cout << res;
return 0;
}
if(n%4 == 0 )
{
cout << n/2;
return 0;
}
cout << -1;
}
