## https://yukicoder.me/problems/no/1008

def solve2(N, M, xw, c):
    array = [0] * (N + 1)
    for x, w in xw:
        d = w // c
        if d > 0:
            from_i = x + 1
            to_i = x + d
            if from_i < N:
                array[from_i] += -c
            if to_i < N:
                array[to_i + 1] -= -c
    a = 0
    for i in range(N + 1):
        a += array[i]
        array[i] = a
    for x, w in xw:
        array[x] += w
        e = w % c
        d = w // c
        end_index = x + d
        if end_index < N:
            array[end_index + 1] -= e
    a = 0
    for i in range(N + 1):
        a += array[i]
        array[i] = a
    return array[:N]

def solve(N, M, A, xw, c):
    if c > 0:
        array_right = solve2(N, M, xw, c)

        xw_ = []
        for x, w in xw:
            xw_.append((N - 1 - x, w))
        array_left = solve2(N, M, xw_, c)
        array_left.reverse()

        array = [0] * N
        for i in range(N):
            array[i] = array_left[i] + array_right[i]
        for x, w in xw:
            array[x] -= w

        for i in range(N):
            if array[i] >= A[i]:
                return False
            
        return True
    else:
        sum_w = 0
        for _, w in xw:
            sum_w += w
        for i in range(N):
            if sum_w >= A[i]:
                return False
        return True


def main():
    N, M = map(int, input().split())
    A = list(map(int, input().split()))

    xw = []
    for _ in range(M):
        x, w = map(int, input().split())
        xw.append((x - 1, w))

    # そもそもcが=sum_wでも壊れることはない?
    cost = [0] * N
    for x, w in xw:
        cost[x] += w
    for i in range(N):
        if cost[i] >= A[i]:
            print(-1)
            return
        
    # cの壊れる最小値を2部探索で計算
    low = 0
    high = max(cost)
    while high - low > 1:
        mid = (high + low) // 2
        if solve(N, M, A, xw, mid):
            high = mid
        else:
            low = mid
    if solve(N, M, A, xw, low):
        print(low)
    else:
        print(high)





if __name__ == "__main__":
    main()