求dalao调代码，修改了排序代码后还是只有20分

#include <stdio.h>

int main()
{
	int l=0,m=0,i,j,t;
	scanf("%d %d",&l,&m);
	l += 1;
	int a[m],b[m];
	int n = 0;
	for(i=0;i<m;i++)
	{
		scanf("%d %d",&a[i],&b[i]);
	}
	for(j=0;j<m;j++)
	{
		for(i=0;i<m-1;i++)
		{
			if(a[i]>a[i+1])
			{
				t=a[i];a[i]=a[i+1];a[i+1]=t;
				t=b[i];b[i]=b[i+1];b[i+1]=t;
			}
		}
	}
	n = b[0]-a[0]+1;
	for(i=0;i<m-1;i++)
	{
		if(a[i+1]<=b[i])
		{
			if(b[i+1]>b[i]){n += (b[i+1]-b[i]);}
		}
		else if(a[i+1]>b[i]){n += (b[i+1]-a[i+1]+1);}
	}
	printf("%d",l-n);
	
	
	return 0;
}