题解

#include<bits/stdc++.h>
using namespace std;
int i,j,m,n,temp;
int a[50001],x[50001],y[50001];
int gs[50001],gs2[50001];

int r()
{
    int ans=0,f=1;
    char ch;
    while(ch<'0'||ch>'9')
    {
        if(ch=='-')
        f=-1;
        ch=getchar();
    }
    while(ch>='0'&&ch<='9')
    {
        ans*=10;
        ans+=ch-'0';
        ch=getchar();
    }
    return ans*f;
}




int main()
{
    n=r();

    for(i=1;i<=n;i++)
    {
        x[i]=r(),y[i]=r();
    }
    a[1]=1,a[2]=x[1];
    for(i=3;i<=n;i++)
    {
        if(x[a[i-1]]==a[i-2]) a[i]=y[a[i-1]];
        else a[i]=x[a[i-1]];
    }
    int le,ri;
    for(i=1;i<=n;i++)
    {
        le=i-1;
        ri=i+1;
        if(!le)
        le=n;
        if(ri==n+1)
        ri=1;
        if((x[a[i]]!=a[ri]||a[le]!=y[a[i]])&&
        (y[a[i]]!=a[ri]||a[le]!=x[a[i]]))
        {
            cout<<-1;
            return 0;
        }
    }
    int dis1,dis2;
    for (i=1; i<=n; i++)
    {
        dis1=(a[i]-i+n)%n;
        dis2=(a[i]+i-1)%n;
        gs[dis1]++;
        gs2[dis2]++;
    }
    int ans=0;
    for (i=0; i<n; i++) 
    {
        if (ans<gs[i]) ans=gs[i];
        if (ans<gs2[i]) ans=gs2[i];
    }
    cout<<n-ans;
    return 0;
}

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