#include < iostream >
using   namespace  std;

int  n,m,x,f[ 1000 ][ 31 ];
int  main()
{
    scanf( " %d%d " , & n, & m);
    memset(f, 0 , sizeof (f));
     for ( int  i = 1 ;i <= n; ++ i){
        scanf( " %d " , & x);  -- x;
        f[i][ 0 ] = f[i - 1 ][ 0 ] + (x == 0 ? 1 : 0 );
         for ( int  j = 1 ;j <= m; ++ j)
            f[i][j] = max(f[i - 1 ][j],f[i - 1 ][j - 1 ]) + (j % 2 == x ? 1 : 0 );   // j%2==x 表示j次移动后奶牛是在那棵数下。能否接到第i可以树。
    }
     int  ans = 0 ;
     for ( int  i = 1 ;i <= m; ++ i)
         if (f[n][i] > ans) ans = f[n][i];
    printf( " %d\n " ,ans);
     return   0 ;
}

滚动数组：
#include < iostream >
#define  max(a,b)(a>b?a:b)
using   namespace  std;

int  main()
{
     int  n,m,x,f[ 2 ][ 31 ];
    scanf( " %d%d " , & n, & m);
    memset(f, 0 , sizeof (f));
     for ( int  i = 1 ;i <= n; ++ i){
        scanf( " %d " , & x);  -- x;
        f[i & 1 ][ 0 ] = f[ 1 - (i & 1 )][ 0 ] + (x == 0 ? 1 : 0 );
         for ( int  j = 1 ;j <= m; ++ j)
            f[i & 1 ][j] = max(f[ 1 - (i & 1 )][j],f[ 1 - (i & 1 )][j - 1 ]) + (j % 2 == x ? 1 : 0 );
    }
    x = 0 ;
     for ( int  i = 0 ;i <= m; ++ i)
         if (f[n & 1 ][i] > x) x = f[n & 1 ][i];
    printf( " %d\n " ,x);
     return   0 ;
}