为什么if(flag=1)执行了if()语句，if(flag==1)反而没有计算if()里的语句

#include <stdio.h>

int main()

{

	int year=2018;

	int month=8;

	int day=8;

	int sum ,flag;

	switch(month)

	{

		case 1:

		sum=31+334;

		break ;

		case 2:

		sum=28+306;

		break ;

		case 3:

		sum=31+275;

		break ;

		case 4:

		sum=30+245;

		break ;

		case 5:

		sum=31+214;

		break ;

		case 6:

		sum=30+184;

		break ;

		case 7:

		sum=153+31;

		break ;

		case 8:

		sum=122+31;

		break ;

		case 9:

		sum=92+30;

		break ;

		case 10:

		sum=61+31;

		break ;

		case 11:

		sum=31+30;

		break;

		case 12:

		sum=31;

		break;

	}

	sum=365-sum+day;

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

	{

	flag=1;

	}

	else

	

		flag=0;

	

	if(flag==1 && month>2)

	{

	     sum=sum+1;

	}

	printf("%d年%d月%d日是该年中的第%d天",year,month,day,sum);

	return 0;

}