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

# %%
N, M = map(int, readline().split())
A = np.array(readline().split(), np.int64)
m = map(int, read().split())
XW = tuple(zip(m, m))


# %%
def test(c):
    if c == 0:
        return sum(w for x, w in XW) <= A.min()

    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

    B1 = np.cumsum(B1_d)[:-1]
    B0 = np.cumsum(B0_d)[:-1]
    cost = B1 * np.arange(N, dtype=np.int64) + B0
    return np.all(cost <= A)


# %%
if any(A[x - 1] < w for x, w in XW):
    print(-1)
    exit()

left = -1
right = 10 * 5 + 10
while left + 1 < right:
    x = (left + right) // 2
    if test(x):
        right = x
    else:
        left = x
print(right)