#!/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)
XW = np.array(read().split(), np.int64)
X = XW[::2] - 1
W = XW[1::2]


# %%
def calc_cost(N, c, X, W):
    B1_d = np.zeros(N + 1, np.int64)
    B0_d = np.zeros(N + 1, np.int64)
    L = X - (W + c - 1) // c + 1
    L = np.maximum(0, L)
    const_term = W - c * X
    np.add.at(B1_d, L, c)
    np.add.at(B0_d, L, const_term)
    np.add.at(B1_d, X+1, -c)
    np.add.at(B0_d, X + 1, -const_term)

    R = X + (W + c - 1) // c - 1
    R = np.minimum(R, N - 1)
    const_term = W + c * X
    np.add.at(B1_d, X + 1, -c)
    np.add.at(B0_d, X + 1, const_term)
    np.add.at(B1_d, R + 1, c)
    np.add.at(B0_d, R + 1, -const_term)
    B0 = np.cumsum(B0_d)[:-1]
    B1 = np.cumsum(B1_d)[:-1]
    return B0 + B1 * np.arange(N, dtype=np.int64)


def test(c):
    if c == 0:
        return W.sum() <= A.min()
    cost = calc_cost(N, c, X, W)
    return np.all(cost < A)


# %%
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)