Codeforces Round #351 (VK Cup 2016 Round 3, Div. 2 Edition) B

#include<stdio.h>
#include<algorithm>
#include<vector>
#include<string.h>
using namespace std;


int main()
{
      int n,m,i;
      while(scanf("%d%d",&n,&m)!=EOF)
      {
          vector<int> da,xi;
          int a,b;
          int maxa=1,minb=n;
          for(i=1;i<=m;i++)
          {
              scanf("%d%d",&a,&b);
              if(a>b)
              {
                  maxa=max(maxa,b);
                  minb=min(minb,a);
              }
              else
              {
                   maxa=max(maxa,a);
                  minb=min(minb,b);
              }
          }
           if(minb<maxa) printf("0\n");
              else printf("%d\n",minb-maxa);
      }
      return 0;
}