include<stdio.h>

int main()
{
int L,m,a,b,left=10000,right=0;
while(scanf("%d %d",&L,&m)!=EOF)
{
for(int i=0;i<m;i++)
{
while(scanf("%d %d",&a,&b)!=EOF)
{
if(left>a)
{
left=a;
}
if(right<b)
{
right=b;
}
}
}
L=L-right+left;
printf("%d",L);
}
}