import sys

input = sys.stdin.buffer.readline


def isOK(c):
    queries = [[] for _ in range(N + 1)]
    for x, w in people:
        left = max(x - (w // c), 0)
        queries[left].append((1, w - c * (x - left)))
        queries[x].append((2,))
        right = min(x + (w // c), N)
        queries[right].append((3, w - c * (right - x)))

    positive = 0
    cur = 0
    for i in range(N):
        cur += positive * c
        for q in queries[i]:
            if q[0] == 1:
                positive += 1
                cur += q[1]
            elif q[0] == 2:
                positive -= 2
            else:
                # assert q[0] == 3
                positive += 1
                cur -= q[1]

        if cur >= A[i]:
            return False
    return True


def solve(N, M, A, people):
    # 耐久度0
    if min(A) > sum(w for _, w in people):
        return 0

    # 不可能
    direct = [0] * N
    for x, w in people:
        direct[x] += w
    for i in range(N):
        if direct[i] >= A[i]:
            return -1

    l, r = 0, 10 ** 9
    while r - l > 1:
        m = (r + l) // 2
        if isOK(m):
            r = m
        else:
            l = m
    return r


N, M = map(int, input().split())
A = tuple(map(int, input().split()))
people = []
for _ in range(M):
    x, w = map(int, input().split())
    x -= 1
    people.append((x, w))
print(solve(N, M, A, people))