output_description: Print the number of ID cards that allow us to pass all the gates alone.
  
  * * *
input_description: Input is given from Standard Input in the following format:
  
      
      
      N M
      L_1 R_1
      L_2 R_2
      \vdots
      L_M R_M
attempt: # -*- coding: utf-8 -*-
  
  debug = False
  
  
  def log(text):
      if debug:
          print(text)
  
  
  def parse_input(lines_as_string=None):
  
      global debug
      lines = []
      if lines_as_string is None:
          debug = False
          lines.append(input())
          token = lines[0].split(" ")
          n = int(token[0])
          m = int(token[1])
          for i in range(m):
              lines.append(input())
      else:
          debug = True
          lines = [e for e in lines_as_string.split("\n")][1:-1]
          token = lines[0].split(" ")
          n = int(token[0])
          m = int(token[1])
  
      lr = []
      for i in range(1, m + 1):
          token = lines[i].split(" ")
          l = int(token[0])
          r = int(token[1])
          lr.append({"l": l, "r": r})
  
      return (n, m, lr)
  
  
  def solve(n, m, lr):
  
      if len(lr) == 1:
          l = lr[0]["l"]
          r = lr[0]["r"]
          return r - l + 1
  
      l = lr[0]["l"]
      r = lr[0]["r"]
      c = {"l": l, "r": r}
      count = 0
      for i in range(1, m):
  
          if lr[i]["l"] <= c["r"] and c["l"] <= lr[i]["r"]:
              c["r"] = min(c["r"], lr[i]["r"])
              c["l"] = max(c["l"], lr[i]["l"])
              count = count + 1
  
          if debug:
              log("c=%s" % c)
  
      result = c["r"] - c["l"] + 1
  
      if debug:
          log("count=%s" % count)
          log("result=%s" % result)
  
      if count == 0:
          return 0
  
      return result
  
  
  def main():
      # 出力
  
      print("%s" % solve(*parse_input()))
  
  
  if __name__ == "__main__":
  
      main()
  
problem_description: Statement
  
  We have N ID cards, and there are M gates.
  
  We can pass the i-th gate if we have one of the following ID cards: the L_i-
  th, (L_i+1)-th, ..., and R_i-th ID cards.
  
  How many of the ID cards allow us to pass all the gates alone?
samples: [{"input": "4 2\n    1 3\n    2 4", "output": "2\n    \n\nTwo ID cards allow us to pass all the gates alone, as follows:\n\n  * The first ID card does not allow us to pass the second gate.\n  * The second ID card allows us to pass all the gates.\n  * The third ID card allows us to pass all the gates.\n  * The fourth ID card does not allow us to pass the first gate.\n\n* * *"}, {"input": "10 3\n    3 6\n    5 7\n    6 9", "output": "1\n    \n\n* * *"}, {"input": "100000 1\n    1 100000", "output": "100000"}]