#include<iostream>
using namespace std;
void move (int A[],int head, int tail){
for( ; head <= tail ; head++ ){
A[head]=0;
}
}
int main( ){
int l = 0 , m = 0;
while(cin >> l >> m){
int* A=(int*)malloc(sizeof(int) * (l+1) ) ;
for(int i = 0 ; i <= l ; i++ ){
A[i]=1;
}
while(m>0){
m--;
int head = 0 ,tail = 0 ;
cin >> head >>tail ;
move(A,head,tail);
}
int sum = 0 ;
for(int i = 0 ; i <= l ; i++ ){
if(A[i]==1){
sum++;
}
}
cout << sum ;
}
return 0;
}