Bridge Construction - Min overlap intervals

#python 3.7.1
import sys

cache = {}
def minOverlap(indx):
  if indx in cache.keys():
    return cache[indx]
  if intervals[indx][0] == 1:
    return intervals[indx][1] - intervals[indx][0] + 1

  t = indx+1
  ans = sys.maxsize
  while(t< m and intervals[t][1] >= intervals[indx][0] - 1):
    ans = min(ans, minOverlap(t))
    t+=1

  cache[indx] = ans + intervals[indx][1] - intervals[indx][0] + 1
  return cache[indx]

n, m = map(int, input().strip().split())
intervals = [list(map(int, input().strip().split())) for i in range(m)]

sections = {k:0 for k in range(1, n+1)}
for a, b in intervals:
  for key in range(a,b+1):
    sections[key] = 1

if len(set(sections.values())) == 2:
  print(-1)
else:
  intervals = sorted(intervals, key = lambda x:x[1], reverse=True)
  ans = minOverlap(0)
  print(ans)