从现在开始，牛的数量组成数列a[n]：
1，1，1，2，3，4，5，7，10，14，19，26。。。（n=0,1,……20）
求a[20]

从a[4]开始有规律a[i]=a[i-1]+a[i-4]

则程序为
#include "stdio.h"
#include "conio.h"
void main()
{
int a[21];
a[0]=1;
printf("now:%d\n",a[0]);
a[1]=1;
printf("1 year after:%d\n",a[1]);
a[2]=1;
printf("2 years after:%d\n",a[2]);
a[3]=2;
printf("3 years after:%d\n",a[3]);
for(int i=4;i<=20;i++)
{
a[i]=a[i-1]+a[i-4];
printf("%d years after:%d\n",i,a[i]);
}
getch();
}

==============================================
结果

now:1
1 year after:1
2 years after:1
3 years after:2
4 years after:3
5 years after:4
6 years after:5
7 years after:7
8 years after:10
9 years after:14
10 years after:19
11 years after:26
12 years after:36
13 years after:50
14 years after:69
15 years after:95
16 years after:131
17 years after:181
18 years after:250
19 years after:345
20 years after:476

 

==============================================

import java.io.BufferedReader;
import java.io.InputStreamReader;

class ReadData
{
 public static InputStreamReader inreader=new InputStreamReader(System.in);
 public static BufferedReader breader=new BufferedReader(inreader);
 public static int readInt()throws Exception
 {
  String str;
  System.out.println("请输入年数:");
  str=breader.readLine();
  str=str.trim();
  return Integer.parseInt(str);
 }
}
public class CountCow
{
 public static void main(String[] args)throws Exception
 {
  ReadData r = new ReadData();
  int n = r.readInt();
  long i = CountCow.fib(n);
  System.out.println(i);
 }
 public static long fib(int n)
 {
  long s = 1;
  if(n==1||n==2||n==3)
  {
   s = 1;
  }
  else
  {
   s = CountCow.fib(n-1) + CountCow.fib(n-3);
  }
  return s;
 }
}