import sys
from collections import defaultdict, Counter, deque
from itertools import permutations, combinations, product, combinations_with_replacement, groupby, accumulate
import operator
from math import sqrt, gcd, factorial
#from math import isqrt, prod, comb  #python3.8用(notpypy)
#from bisect import bisect_left, bisect_right
#from functools import lru_cache, reduce
#from heapq import heappush, heappop, heapify, heappushpop, heapreplace
#import numpy as np
#import networkx as nx
#from networkx.utils import UnionFind
#from numba import njit, b1, i1, i4, i8, f8
#numba例 @njit(i1(i4[:], i8[:, :]),cache=True) 引数i4配列、i8 2次元配列,戻り値i1
#from scipy.sparse import csr_matrix
#from scipy.sparse.csgraph import shortest_path, floyd_warshall, dijkstra, bellman_ford, johnson, NegativeCycleError, maximum_bipartite_matching, maximum_flow, minimum_spanning_tree
def input(): return sys.stdin.readline().rstrip()
def divceil(n, k): return 1+(n-1)//k  # n/kの切り上げを返す
def yn(hantei, yes='Yes', no='No'): print(yes if hantei else no)


def main():
    mod = 10**9+7
    mod2 = 998244353
    n,k=map(int, input().split())
    rc=[list(map(int, input().split())) for i in range(k)]
    use=[0]*k
    dic={tuple(a):i for i,a in enumerate(rc)}
    ans=1
    for i,a in enumerate(rc):
        r,c=a
        if use[i]:
            continue
        if r==c:
            print(-1)
            return
        ru,cu=r,c
        while (ru,cu) in dic:
            use[dic[(ru,cu)]]=0
            ru+=1
            cu+=1
        rd,cd=r,c
        while (rd,cd) in dic:
            use[dic[(rd,cd)]]=0
            rd-=1
            cd-=1
        ans=max(ans,cu-cd)
    print(ans)
        
if __name__ == '__main__':
    main()