#include <stdio.h>
void swap (int *a,int *b);
int main ()
{
	int n,m,i,a,b,k,sum;
	scanf ("%d%d",&n,&m);
	int arr[1000],brr[1000];
	for (i = 0;i<m;i++)
	scanf ("%d%d",&arr[i],&brr[i]);
	for (i = 0;i<m-1;i++)
	 for (k = 0;k<m-k-1;k++)
	   {
	   	if (arr[k]>arr[k+1])
	   	{
	   		swap (&arr[k],&arr[k+1]);
	   		swap (&brr[k],&brr[k+1]);
		   }
	   }
	for (i = 0,sum = 0;i<m-1;i++)
	{
		if (brr[i]<=arr[i+1])
		sum =sum + 1+brr[i]-arr[i];
		else if (brr[i]<=brr[i+1])
	        arr[i+1] = arr[i];
	          else
	            {
	            	arr[i+1] = arr[i];
	            	brr[i+1]=brr[i];
				 } 
	}
	printf ("%d",n+1-sum-brr[i]+arr[i]-1);
	return 0;
	   
}
void swap (int *a,int *b)
{
	int temp;
	temp = *a;
	*a = *b;
	*b = temp;
}

看了看题解，大一新生连树是啥都不知道_(:з」∠)_