module graph_mod
    implicit none
    type node
        integer(4),allocatable:: to(:)
        integer(4):: l
    end type
    
contains
    subroutine init(n)
        type(node):: n
        allocate(n%to(1))

        n%l = 0
    end subroutine


    subroutine regist(n,m)
        type(node):: n
        integer(4):: m

        if (n%l+1 > size(n%to)) call add_(n)
        n%l=n%l+1
        n%to(n%l) = m
    end subroutine


    subroutine add_(n)
        type(node):: n
        integer(4),allocatable:: tmp(:)
        integer(4):: l

        l = size(n%to)
        allocate(tmp(l))
        tmp(:) = n%to(:)
        deallocate(n%to)
        allocate(n%to(l*2))
        n%to(1:l) = tmp(:)
        deallocate(tmp)
    end subroutine
end module

module deque_mod
    use,intrinsic :: iso_fortran_env
    implicit none
    type deque
        integer(int32),pointer:: v(:)
        integer(int32):: l,r
        integer(int32):: lmax, rmax
    end type
    private
    public:: deque
    public:: init_deque
    public:: append, appendleft
    public:: pop, popleft
    public:: right, left
    public:: remaining_elements
    public:: remain

contains
    subroutine init_deque(dq)
        ! 初期化
        type(deque):: dq
        dq%r=0; dq%rmax=1
        dq%l=1; dq%lmax=-1
        allocate(dq%v(dq%lmax:dq%rmax))
    end subroutine


    subroutine append(dq,num)
        ! 右端に挿入
        type(deque):: dq
        integer(int32):: num
        if (dq%r+1 > dq%rmax) call add_(dq)
        dq%r=dq%r+1
        dq%v(dq%r) = num
    end subroutine

    subroutine appendleft(dq,num)
        ! 左端に挿入
        type(deque):: dq
        integer(int32):: num
        if (dq%l-1 < dq%lmax) call add_(dq)
        dq%l=dq%l-1
        dq%v(dq%l) = num
    end subroutine

    subroutine add_(dq)
        ! arrayの延長
        type(deque):: dq
        integer(int32):: l
        integer(int32),pointer:: tmp(:)
        l = size(dq%v)
        allocate(tmp(l))
        tmp(:) = dq%v(:)
        deallocate(dq%v)
        allocate(dq%v(2*dq%lmax:2*dq%rmax))
        dq%v(dq%lmax:dq%rmax) = tmp(:)
        dq%lmax = 2*dq%lmax
        dq%rmax = 2*dq%rmax
    end subroutine


    function pop(dq) result(ret)
        ! 右端から取り出し
        type(deque):: dq
        integer(int32):: ret
        ret = dq%v(dq%r)
        dq%r=dq%r-1
    end function

    function popleft(dq) result(ret)
        ! 左端から取り出し
        type(deque):: dq
        integer(int32):: ret
        ret = dq%v(dq%l)
        dq%l=dq%l+1
    end function


    function right(dq) result(ret)
        ! 右端を確認
        type(deque):: dq
        integer(int32):: ret
        ret = dq%v(dq%r)
    end function

    function left(dq) result(ret)
        ! 左端を確認
        type(deque):: dq
        integer(int32):: ret
        ret = dq%v(dq%l)
    end function

    function remaining_elements(dq) result(ret)
        ! 残りの要素数
        type(deque):: dq
        integer(int32):: ret 
        ret = dq%r - dq%l + 1
    end function

    function remain(dq) result(ret)
        ! 要素が残っているかどうか
        type(deque):: dq
        logical:: ret
        ret = remaining_elements(dq) > 0
    end function
end module

program name
    use,intrinsic :: iso_fortran_env
    use:: graph_mod
    use:: deque_mod
    implicit none
    integer(int32):: n,m,a,b,i,now, dst
    integer(int32), allocatable:: dtc(:)
    type(node),allocatable:: g(:)
    type(deque):: todo, ks
    logical,allocatable:: see(:)

    read*, n,m
    allocate(g(n),see(n),dtc(n))
    do i=1,n
        call init(g(i))
    end do
    do i=1,m
        read*, a,b
        call regist(g(a),b)
        call regist(g(b),a)
    end do
    call init_deque(todo)
    call init_deque(ks)

    call append(todo,1)
    call append(ks,0)
    dtc(:) = 2000000
    dtc(1) = 0
    see(:) = .false.
    do while(remain(todo))  
        now = popleft(todo)
        see(now) = .true.
        do i=1,g(now)%l
            dst = g(now)%to(i)
            dtc(dst) = min(dtc(dst),dtc(now)+1)
            if (.not. see(dst)) then
                call append(todo, dst)
            end if
        end do
    end do

    if (dtc(n) <= 2) then
        print'(a)', 'POSSIBLE'
    else
        print'(a)', 'IMPOSSIBLE' 
    end if
end program name

Measured memory_bytes: 26680