from itertools import accumulate
import sys
input = sys.stdin.buffer.readline
sys.setrecursionlimit(10 ** 7)

N, M = map(int, input().split())
A = tuple(map(int, input().split()))
XW = tuple(tuple(map(int, input().split())) for _ in range(M))
X, W = zip(*XW)
wsum = sum(W)
if all(a > wsum for a in A):
    print(0)
    exit()


def C(k):
    dpR = [0] * N
    dpL = [0] * N
    for x, w in XW:
        x -= 1
        d = (w + k - 1) // k
        if x + 1 < N:
            dpR[x + 1] -= k
            if x + d < N:
                dpR[x + d] += k
        if N - 1 - (x - 1) < N:
            dpL[N - 1 - (x - 1)] -= k
            if N - 1 - (x - d) < N:
                dpL[N - 1 - (x - d)] += k
    dpR = list(accumulate(dpR))
    dpL = list(accumulate(dpL))
    for x, w in XW:
        x -= 1
        d = (w + k - 1) // k
        dpR[x] += w
        dpL[N - 1 - x] += w
        if x + d < N:
            dpR[x + d] -= w
            dpR[x + d] += k * (d - 1)
        if N - 1 - (x - d) < N:
            dpL[N - 1 - (x - d)] -= w
            dpL[N - 1 - (x - d)] += k * (d - 1)
    dpR = list(accumulate(dpR))
    dpL = list(accumulate(dpL))
    dpL.reverse()
    B = [l + r for l, r in zip(dpL, dpR)]
    for x, w in XW:
        B[x - 1] -= w
    return all(b < a for a, b in zip(A, B))


inf = max(W) + 10
l = 0
r = inf
while r - l > 1:
    m = (r + l) // 2
    if C(m):
        r = m
    else:
        l = m

# print(l, r)
if r >= inf:
    print(-1)
else:
    print(r)