function main(input) {
  var input = input.trim().split("\n");
  var N = input[0].split(" ")[0]-0;
  var M = input[0].split(" ")[1]-0;
  var list = [];
  var max_l = 0;
  for(var i=1;i<=M;i++){
    var l = input[i].split(" ")[0]-0;
    var r = input[i].split(" ")[1]-0;
    list.push({p:'l', n:l});
    list.push({p:'r', n:r});
    max_l = Math.max(max_l, l);
  }
  list.sort((a,b)=>{
    if(a.n!=b.n) return a.n-b.n;
    else if(a.p=='l') return -1;
    else return 1;
   });
  var ans = 0;
  var tmp = 0;
  for(var i=0,e=list.length;i<e;i++){
    if(list[i].p == 'l') tmp++;
    if(list[i].p == 'r') tmp--;
    if(tmp==M) ans+=list[i+1].n-list[i].n+1;
    if(max_l == list[i].n) break;
  }
  console.log(ans);
}

main(require('fs').readFileSync('/dev/stdin', 'utf8'))
