学步园

#include<iostream>
#include<cstring>
#include<cstdio>
using namespace std;
inline int read(){  
    int x=0,f=1;char ch=getchar();  
    while(ch<'0'||ch>'9'){if(ch=='-')f=-1;ch=getchar();}  
    while(ch>='0'&&ch<='9'){x=x*10+ch-'0';ch=getchar();}  
    return x*f;  
}  
int n,ans=99999999,d[30001],f[30001][4];
void dp(){
    memset(f,127,sizeof(f));
    f[0][1]=f[0][2]=f[0][3]=0;
    for(int i=1;i<=n;i++)
        for(int j=1;j<=3;j++)
            for(int k=1;k<=j;k++){
                if(d[i]==j)f[i][j]=min(f[i][j],f[i-1][k]);
                else f[i][j]=min(f[i][j],f[i-1][k]+1);
            }
    ans=min(ans,f[n][1]);
    ans=min(ans,f[n][2]);
    ans=min(ans,f[n][3]);
}
int main(){
    n=read();
    for(int i=1;i<=n;i++)d[i]=read();
    dp();
    for(int i=1;i<=n>>1;i++)swap(d[i],d[n-i+1]);
    dp();
    printf("%d",ans);
    return 0;
}