#!/usr/bin/env python3
# %%
import sys
read = sys.stdin.buffer.read
readline = sys.stdin.buffer.readline
readlines = sys.stdin.buffer.readlines
import itertools

# %%
N, M = map(int, readline().split())
A = list(map(int, readline().split()))
m = map(int, read().split())
XW = tuple(zip(m, m))


# %%
def calc_cost(N, c, XW):
    B1_d = [0] * (N + 1)
    B0_d = [0] * (N + 1)
    for x, w in XW:
        x -= 1
        L = x - (w + c - 1) // c + 1
        if L < 0:
            L = 0
        const_term = w - c * x
        B1_d[L] += c
        B0_d[L] += const_term
        B1_d[x + 1] -= c
        B0_d[x + 1] -= const_term
        R = x + (w + c - 1) // c - 1
        const_term = w + c * x
        if R >= N:
            R = N - 1
        B1_d[x + 1] -= c
        B0_d[x + 1] += const_term
        B1_d[R + 1] += c
        B0_d[R + 1] -= const_term
    B0 = list(itertools.accumulate(B0_d))[:-1]
    B1 = list(itertools.accumulate(B1_d))[:-1]
    cost = [a * i + b for i, (a, b) in enumerate(zip(B1, B0))]
    return cost


def test(c):
    if c == 0:
        return sum(w for x, w in XW) <= min(A)
    cost = calc_cost(N, c, XW)
    return all(x < a for a, x in zip(A, cost))


# %%
left = -1
right = 10 ** 5 + 10
while left + 1 < right:
    mid = (left + right) // 2
    if test(mid):
        right = mid
    else:
        left = mid

if right > 10 ** 5:
    print(-1)
else:
    print(right)