输入一个年月日，判断是这一年的第几天？

Console.WriteLine("请输入年：");

int year = int.Parse(Console.ReadLine());

Console.WriteLine("请输入月：");

int month = int.Parse(Console.ReadLine());

Console.WriteLine("请输入日：");

int day = int.Parse(Console.ReadLine());

int sum=0;

if ((year % 400 == 0) || ((year % 100 != 0) && (year % 4 == 0)))

{

switch (month)

{

case 1:

sum =day;

break;

case 2:

sum =day + 31;

break;

case 3:

sum =day + 31 + 29;

break;

case 4:

sum =day + 31 + 29 + 31;

break;

case 5:

sum =day + 31 + 29 + 31 + 30;

break;

case 6:

sum =day + 31 + 29 + 31 + 30 + 31;

break;

case 7:

sum =day + 31 + 29 + 31 + 30 + 31 + 30;

break;

case 8:

sum =day + 31 + 29 + 31 + 30 + 31 + 30 + 31;

break;

case 9:

sum =day + 31 + 29 + 31 + 30 + 31 + 30 + 31 + 31;

break;

case 10:

sum =day + 31 + 29 + 31 + 30 + 31 + 30 + 31 + 31 + 30;

break;

case 11:

sum =day + 31 + 29 + 31 + 30 + 31 + 30 + 31 + 31 + 30 + 31;

break;

case 12:

sum =day + 31 + 29 + 31 + 30 + 31 + 30 + 31 + 31 + 30 + 31 + 30;

break;

default:

Console.WriteLine("输入错误");

break;

}

}

else

{

switch (month)

{

case 1:

sum =day;

break;

case 2:

sum =day + 31;

break;

case 3:

sum = day + 31 + 28;

break;

case 4:

sum =day + 31 + 28 + 31;

break;

case 5:

sum =day + 31 + 28 + 31 + 30;

break;

case 6:

sum =day + 31 + 28 + 31 + 30 + 31;

break;

case 7:

sum =day + 31 + 28 + 31 + 30 + 31 + 30;

break;

case 8:

sum =day + 31 + 28 + 31 + 30 + 31 + 30 + 31;

break;

case 9:

sum =day + 31 + 28 + 31 + 30 + 31 + 30 + 31 + 31;

break;

case 10:

sum =day + 31 + 28 + 31 + 30 + 31 + 30 + 31 + 31 + 30;

break;

case 11:

sum =day + 31 + 28 + 31 + 30 + 31 + 30 + 31 + 31 + 30 + 31;

break;

case 12:

sum =day + 31 + 28 + 31 + 30 + 31 + 30 + 31 + 31 + 30 + 31 + 30;

break;

default:

Console.WriteLine("输入错误");

break;

}

}

Console.WriteLine("这是{0}年的第{1}天",year,sum);

判断是这一年的第几天？