POJ 2665

#include<iostream>
#include<stdio.h>
using namespace std;

int main()
{
    //freopen("acm.acm","r",stdin);
    int tree;
    int num;
    int sum;
    int a,b;
    while(1)
    {
    cin>>tree;
    cin>>num;
    if(!tree && !num)
        break;
    ++ tree;
    sum = 0;
    while(num --)
    {
        cin>>a;
        cin>>b;
        sum += b - a + 1;
    }
    cout<<tree - sum<<endl;
    }
}