学步园

//给出一段距离，即是树木的总数，然后再给出几个树木需要砍伐或者移走的区域，求剩下的树木数！注意：
//That's to say that there used to be 301 trees on the road. Now the section from 100m to 200m 
//is assigned for subway station, so 101 trees need to be moved away and only 200 trees are left. 
#include <iostream>
using namespace std;


int main()
{
    int l, m, i, s1, s2, ans, tmp;
    while (cin >> l >> m){
          if (l == 0 && m == 0)  break;
          ans = 0; 
          for (i = 0; i < m; i++){ 
              cin >> s1 >> s2;
              tmp = s2 - s1;
              ans += tmp + 1; 
          } 
          ans = l + 1 - ans;
          cout << ans << endl; 
    } 
    system("pause"); 
}