# coding: utf-8
# Your code here!

import sys
readline = sys.stdin.readline
read = sys.stdin.read

n,m = [int(i) for i in readline().split()]
a = [int(i) for i in readline().split()]

xw = [[int(i) for i in readline().split()] for _ in range(m)]
V = 0
for i in range(m):
    xw[i][0] -= 1
    V += xw[i][1]

if V < min(a):
    print(0)
    exit()

from itertools import accumulate
def check(c): #耐久値 x で耐えられるか？
    imos = [0]*(n)
    res = [0]*(n)
    for x,w in xw:
        res[x] += w
        v = w//c

        l = max(0,x-v)
        r = x-1
        #print(l,r)
        if l < r:
            X = w-(x-l)*c
            y = w-c
            imos[l] += X
            imos[l+1] += c-X
            if r+1 < n:
                imos[r+1] += -y-c
            if r+2 < n:
                imos[r+2] += y
        elif l==r:
            res[r] += w-c
        
        r = min(n-1,x+v)
        l = x+1
        #print(l,r)
        if l < r:
            X = w-c
            y = w-c*(r-x)
            imos[l] += X
            imos[l+1] += -c-X
            if r+1 < n:
                imos[r+1] += -y+c
            if r+2 < n:
                imos[r+2] += y
        elif l==r:
            res[r] += w-c
        
        #print(x,w,c)
        #print(imos)
        #imos = [0]*n
    imos = accumulate(imos)    
    imos = list(accumulate(imos))
    #print(imos,res,a)
    return all(imos[i]+res[i] < a[i] for i in range(n))    


#print(check(2))
ng = 0
ok = M = 10**6
while ok-ng > 1:
    mid = (ok+ng)//2
    if check(mid):
        ok = mid
    else:
        ng = mid


if ok < M: 
    print(ok)
else:
    print(-1)