// 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;

}