import java.io.*
import java.util.*
import java.lang.*
import java.lang.Math.*

class CumSum(val A: LongArray) {
    private var csum: LongArray = LongArray(A.size + 1){0L}
    init {
        for (i in 1..A.size) {
            this.csum[i] = this.csum[i-1] + A[i-1]
        }
    }
    fun sum(L: Int, R: Int): Long {
        val r = min(R, A.size)
        val l = max(L, 0)
        if (l >= r) return 0
        return this.csum[r] - this.csum[l]
    }
}

fun solve() {
    val N = rd.readInt()
    val A = rd.readLongArray()

    if (N==0) {
        if (A[0]!=1L) {
            println(-1)
        } else {
            println(1)
        }
        return
    }

    val Lim = Array<Long>(N+1){LINF}
    for (d in 0 .. min(N, 60)) {
        Lim[d] = (1L shl d)
    }

    val B = LongArray(N+1){0L}
    B[0] = if (A[0]==0L) 1L else 0L
    val cs = CumSum(A)
    for (d in 1 .. N) {
        B[d] = min(B[d-1]*2, cs.sum(d,N+1)) - A[d]

        if (B[d]<0
            || B[d-1] == 0L && A[d] > 0L
            || A[d]+B[d] > Lim[d]) {
            println(-1)
            return
        }
    }

    val ans = B.sum() + A.sum()
    println(ans)
}


//val rd = debug.Reader("etc", "nomura2020", "c", "sample-1")
val rd = Reader()


fun main(args: Array<String>) {
    try {
        var exception: Throwable? = null
        val sth = Thread(null, SolveThread(), "solveThread", 128 * 1024 * 1024)
        sth.setUncaughtExceptionHandler { _, e -> exception = e }
        sth.start(); sth.join()
        if (exception != null) throw exception!! }
    catch (e: Exception) { throw e }
    finally { rd.close() }
}
const val MOD = 1_000_000_007L; const val INF = Int.MAX_VALUE/2; const val LINF = Long.MAX_VALUE/2
const val EPS = 1e-9; fun DBLEQ(a: Double, b: Double): Boolean {return abs(a - b) < EPS}
data class IntPair(val first: Int, val second: Int)
data class To(val idx: Int, val cost: Long)
typealias ALI = ArrayList<Int>
fun <T>edge(V: Int): Array<ArrayList<T>> { return Array(V){ArrayList<T>()} }
val errPrintln: (Any) -> Unit = { msg -> System.err.println(msg) }
fun rAssert(exp: Boolean, lazyMsg: () -> String = {""}) {
    if (!exp) { val msg = lazyMsg(); errPrintln("EAssert Error: $msg"); throw Error(msg) }
}
fun tAssert(exp: Boolean, lazyMsg: () -> String = {""}) {
    if (!exp) { val msg = lazyMsg(); errPrintln("TAssert Error: $msg"); Thread.sleep(3000) }
}
class SolveThread : Runnable {
    override fun run() { solve() }
}
class Reader {
    private val br = BufferedReader(InputStreamReader(System.`in`))
    val readString: () -> String = { br.readLine() }
    val readInt: () -> Int = { br.readLine().toInt() }
    val readLong: () -> Long = { br.readLine().toLong() }
    val readIntArray: () -> IntArray = { br.readLine().split(' ').map { it.toInt() }.toIntArray() }
    val readLongArray: () -> LongArray = { br.readLine().split(' ').map { it.toLong() }.toLongArray() }
    val readListInt: () -> List<Int> = { br.readLine().split(' ').map { it.toInt() } }
    val readListLong: () -> List<Long> = { br.readLine().split(' ').map { it.toLong() } }
    val readListIntCol: (Int) -> List<Int> = { N -> (0 until N).map { readInt() } }
    val readListIntCols: (Int) -> List<List<Int>> = { N -> (0 until N).map { br.readLine().split(' ').map { it.toInt() } } }
    val readListLongCol: (Int) -> List<Long> = { N -> (0 until N).map { readLong() } }
    val readListLongCols: (Int) -> List<List<Long>> = { N -> (0 until N).map { br.readLine().split(' ').map { it.toLong() } } }
    val close: () -> Unit = { br.close() }
    protected fun finalize() { close() }
}


Measured memory_bytes: 57656