【codevs1106】 篝火晚会

// codevs1106
#include<algorithm>
#include<iostream>
#include<cstdlib>
#include<cstring>
#include<cstdio>
#include<cmath>
#define LL long long
#define inf 1<<30
#define Pi acos(-1.0)
#define free(a) freopen(a".in","r",stdin),freopen(a".out","w",stdout);
using namespace std;

const int maxn=50010;
int vis[maxn],t1[maxn],t2[maxn],a[maxn],b[maxn],c[maxn],n;

int main() {
	scanf("%d",&n);
	for (int i=1;i<=n;i++) scanf("%d%d",&a[i],&b[i]);
	c[1]=1;c[2]=a[1];
	vis[c[1]]=vis[c[2]]=1;
	for (int i=2;i<n;i++) {
		if (c[i-1]==a[c[i]]) c[i+1]=b[c[i]];
		else if (c[i-1]==b[c[i]]) c[i+1]=a[c[i]];
		else {puts("-1");return 0;}
		vis[c[i+1]]=1;
	}
	for (int i=1;i<=n;i++) if (!vis[i]) {puts("-1");return 0;}
	int ans=1;
	for (int i=1;i<=n;i++) {
		int t=(c[i]-i+n)%n;
		t1[t]++;
		ans=max(ans,t1[t]);
		t=(c[n-i+1]-i+n)%n;
		t2[t]++;
		ans=max(ans,t2[t]);
	}
	printf("%d",n-ans);
	return 0;
}