C#求该日期是这一年中的第几天

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication1
{
class Program
{
static void Main(string[] args)
{
int years, moth, day,sum=0;
int[] monthDays = { 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 };
years = int.Parse(Console.ReadLine());
moth = int.Parse(Console.ReadLine());
day = int.Parse(Console.ReadLine());
if (years % 400 == 0 || (years % 100 == 0 && years % 4 != 0))
{
monthDays[1] = 29;
}
Console.Write("该天是该年的第");
for (int i = 0; i < moth; i++)
{
sum += monthDays[i];
}
Console.Write(sum + day);
Console.WriteLine("天。");
Console.ReadLine();
}
}
}