import sys

input = sys.stdin.buffer.readline


def accumulate(X):
    Y = [0] * len(X)
    Y[0] = X[0]
    for i in range(1, len(X)):
        Y[i] = Y[i - 1] + X[i]
    return Y


def isOK(c):
    imos = [0] * (N + 2)
    for x, w in people:
        left = max(x - (w // c), 0)
        imos[left] += w - c * (x - left)
        imos[left + 1] += c - (w - c * (x - left))
        imos[x + 1] -= 2 * c
        right = min(x + (w // c), N - 1)
        imos[right + 1] += c - (w - c * (right - x))
        imos[right + 2] += w - c * (right - x)

    B = accumulate(accumulate(imos))
    for i in range(N):
        if B[i] >= A[i]:
            return False
    return True


def solve(N, 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, A, people))