@Test
    fun generate() {
        val random = Random(123)
        for (i in 1..100000) {
            val xs = listOf(S.Plus(random.nextInt(10))) + (1..5).map { S.rands(random) }
            val c = calc(xs)
            if (c != null) {
                val ss = xs.joinToString { wraps(it) }
                println("$ss = $c")
            }
        }
    }

    fun calc(xs: List<S>): Int? {
        var sum: Int? = 0
        for (x in xs) {
            sum = when (x) {
                is S.Plus -> sum?.plus(x.n)
                is S.Minus -> sum?.minus(x.n)
                is S.Mul -> {
                    if (sum == null) null
                    else {
                        val y = sum * x.n
                        if (y < -20 || y > 20) null else y
                    }
                }
                is S.Div -> {
                    if (sum == null) null
                    else {
                        if (x.n <= 0 || (sum / x.n) * x.n != sum) null else sum / x.n
                    }
                }
            }
            if (sum == null)
                break
        }
        return sum
    }

}

sealed class S {
    data class Plus(val n: Int) : S()
    data class Minus(val n: Int) : S()
    data class Mul(val n: Int) : S()
    data class Div(val n: Int) : S()
    companion object {
        fun rands(r: Random): S {
            val x = r.nextInt(10)
            return when (r.nextInt(4)) {
                0 -> Plus(x)
                1 -> Minus(x)
                2 -> Mul(x)
                else -> Div(x)
            }
        }
        fun wraps(s: S): String {
            return when (s) {
                is Plus  -> "+" + s.n
                is Minus -> "-" + s.n
                is Mul   -> "*" + s.n
                is Div   -> "/" + s.n
            }
        }
    }
}