import java.util.*

var g = Array(0){IntArray(0)}
var rg = Array(0){IntArray(0)}

fun main(args: Array<String>) = IO().exec {
    val n = int()
    val m = int()
    val a = IntArray(m)
    val b = IntArray(m)
    for (i in m) {
        a[i] = int()-1
        b[i] = int()-1
    }
    g = mkDGraph(n, a, b)
    rg = mkDGraph(n, b, a)

    var ret: MutableList<Int>? = null
    for (v in n) {
        val prev = IntArray(n){-1}
        val dist = IntArray(n){-1}
        dist[v] = 0
        val que = ArrayDeque<Int>()
        que.offer(v)
        while (true) {
            val cur = que.poll() ?: break
            for (next in g[cur]) {
                if (next == v) {
                    val lst = mutableListOf(cur)
                    var p = cur
                    while (true) {
                        if (p == next) break
                        p = prev[p]
                        lst.add(p)
                    }
                    lst.reverse()
                    if (ret == null || ret.size > lst.size)
                        ret = lst
                }
                if (dist[next] == -1) {
                    prev[next] = cur
                    que.offer(next)
                    dist[next] = dist[cur] + 1
                }
            }
        }
    }

    if (ret != null) {
        println(ret.size)
        for (c in ret) println(c+1)
        return@exec
    }

    println(-1)
}

var state = IntArray(0)

fun mkDGraph(n: Int, from: IntArray, to: IntArray): Array<IntArray> {
    val ret: Array<IntArray> = Array(n){IntArray(0)}
    val cnt = IntArray(n)
    for (f in from) cnt[f]++
    for (i in 0 until n) ret[i] = IntArray(cnt[i])
    for (i in from.indices)
        ret[from[i]][--cnt[from[i]]] = to[i]
    return ret
}

// region template
operator fun Int.iterator() = 0.until(this).iterator()

class IO {
    val printable = 33..126
    val buffer = ByteArray(1024)
    var ptr = 0
    var buflen = 0
    val out = java.io.PrintWriter(System.out)
    fun hasNextByte(): Boolean = if (ptr < buflen) true else {
        ptr = 0
        buflen = System.`in`.read(buffer)
        buflen > 0
    }

    fun readByte(): Byte = if (hasNextByte()) buffer[ptr++] else -1
    fun hasNext(): Boolean {
        while (hasNextByte() && buffer[ptr] !in printable) ptr++
        return hasNextByte()
    }

    fun string(): String {
        if (!hasNext()) throw java.util.NoSuchElementException()
        val sb = StringBuilder()
        var b = readByte()
        while (b in printable) {
            sb.appendCodePoint(b.toInt())
            b = readByte()
        }
        return sb.toString()
    }

    fun long(): Long {
        if (!hasNext()) throw java.util.NoSuchElementException()
        var n = 0L
        var minus = false
        var b = readByte()
        if (b == '-'.toByte()) {
            minus = true
            b = readByte()
        }
        if (b !in '0'.toByte()..'9'.toByte()) {
            throw NumberFormatException()
        }
        while (true) {
            if (b in '0'.toByte()..'9'.toByte()) {
                n *= 10
                n += b - '0'.toByte()
            } else if (b == (-1).toByte() || b !in printable) {
                return if (minus) -n else n
            } else {
                throw NumberFormatException()
            }
            b = readByte()
        }
    }

    fun int(): Int {
        val nl = long()
        if (nl !in Integer.MIN_VALUE..Integer.MAX_VALUE) throw NumberFormatException()
        return nl.toInt()
    }

    fun double(): Double = string().toDouble()
    fun print(obj: Any) = out.print(obj)
    fun print(i: Int) = out.print(i)
    fun print(l: Long) = out.print(l)
    fun println(obj: Any) = out.println(obj)
    fun println(i: Int) = out.println(i)
    fun println(l: Long) = out.println(l)
    inline fun exec(block: IO.() -> Unit) {
        block()
        out.flush()
    }

    inline fun println(block: IO.() -> Any) {
        println(block())
        out.flush()
    }
}
// endregion

Measured memory_bytes: 49860