import os
import sys
import heapq

# 请在此输入您的代码

if __name__=="__main__":
  x,n= map(int,input().split())
  l=[]
  a=[0]*n
  b=[0]*n
  c=[0]*n
  q=[]
  for i in range(n):
    l.append(list( map( int ,input().split()) ))
  l.sort(key=lambda pair:-pair[1])
  total=0
  j=0
  for i in range(x,0,-1):
    while j<n and l[j][1]>=i:
      heapq.heappush(q,l[j])
      j+=1
    if not q:
      print(-1)
      break
    pop = heapq.heappop(q)
    total+=pop[0]
    if pop[2]>1:
      pop[2]-=1
      heapq.heappush(q,pop)
  else:
    print(total)

import os
import sys

# 请在此输入您的代码


def check(t)->bool:
  mylist= [] 
  for i in range(n):
    if t>=s[i]:
      mylist.append([ l[i]-(t-s[i]) ,l[i]+(t-s[i]) ])
  if not mylist:
    return False
  mylist.sort()
  if mylist[0][0]>1:
    return False
  right=mylist[0][1]
  for i in range(1,len(mylist)):
    if mylist[i][0] <= right + 1:
      right= max(right,mylist[i][1])
    else:
      break
  return right>=length

if __name__=="__main__":
  n,length=map(int,input().split())
  l=[0]*n
  s=[0]*n
  for i in range(n):
    l[i],s[i] =  map(int,input().split())
  left=0
  r=2_000_000_000
  while(left<r):
    mid=(left+r)//2
    if check(mid):
      r=mid
    else:
      left=mid+1
  print(r)