import java.io.*;
import java.util.*;

class Main{
    final static long INF = Long.MAX_VALUE / 2;
    final static int MOD = 1_000_000_007;
    final static int SIZE = 1_000_000;
    long[] fac = new long[SIZE];
    long[] inv = new long[SIZE];
    long[] finv = new long[SIZE];
    FastScanner sc = new FastScanner();

    public static void main(String[] args) {
        new Main().solve();
    }

    void solve(){

        int n = sc.nextInt();
        long[] a = new long[n + 1];

        for(int i = 0; i < n + 1; i++){
            a[i] = sc.nextLong();
        }

        if(a[0] > 1 || (a[0] == 1 && n > 1)){ // a[0] が1以下でない または 深さ1以上で a[0] が1 の時 -1
            System.out.println(-1);
            System.exit(0);
        }

        // long b = 2;
        //
        // for(int i = 1; i < n + 1; i++){
        //     if(a[i] > b - a[i - 1] * 2){
        //         System.out.println(-1);
        //         System.exit(0);
        //     }
        //     b *= 2;
        //     if(b > 1000000000) break;
        // }

        long[] p = new long[n + 1]; // p[i] : 親になれるノードの数
        if(a[0] == 1) p[0] = 0;
        else p[0] = 1;

        for(int i = 1; i < n + 1; i++){
            p[i] = p[i - 1] * 2 - a[i]; // 深さiの親になれるノードの数 = 深さi - 1の親になれるノードの数 * 2 - 深さiの葉の数 ?
        }

        long[] m = new long[n + 1]; // m[i] : 深さiの頂点の最大数
        m[0] = 1;

        for(int i = 1; i < n + 1; i++){
            m[i] = p[i - 1] * 2; // 深さiの頂点の最大数 = 深さi - 1の親になれるノードの数 * 2 ?
        }

        long[] s = new long[n + 1]; // s[i] : a[i], a[i + 1], ... a[n] の累積
        s[n] = a[n];

        for(int i = n - 1; i >= 0; i--){
            s[i] = s[i + 1] + a[i];
        }

        long sum = 0;

        for(int i = 0; i < n + 1; i++){
            if(m[i] >= 0){
                sum += Math.min(m[i], s[i]);
                if(a[i] > m[i]){
                    System.out.println(-1);
                    System.exit(0);
                }
            }else{
                sum += s[i];
            }
        }

        System.out.println(sum);

    }

    long gcd(long a, long b){ // return aとbの最大公約数
        if(b == 0){
          return a;
        }
        return gcd(b, a % b);
    }

    long lcm(long a, long b){ // return aとbの最小公倍数
        return a * b / gcd(a, b);
    }

    long inv(long a){ // return aの逆元 (mod MOD)
        return pow(a, MOD - 2);
    }

    long pow(long a, long r){ // return a^r (mod MOD)
        long sum = 1;
        while(r > 0){
            if((r & 1) == 1){ // 2進数表記で末尾1の時
                sum *= a;
                sum %= MOD;
            }
            a *= a;
            a %= MOD;
            r >>= 1;
        }
        return sum;
    }

    long modFact(long n){ // retur n! (mod MOD)
        if(n == 0){
            return 1;
        }
        return n * modFact(n - 1) % MOD;
    }

    long fact(long n){ // return n!
        if(n == 0){
            return 1;
        }
        return n * fact(n - 1);
    }

    void initCOMB(){
        fac[0] = fac[1] = 1;
        inv[1] = 1;
        finv[0] = finv[1] = 1;
        for(int i = 2; i < SIZE; i++){
            fac[i] = fac[i - 1] * i % MOD;
            inv[i] = MOD - inv[MOD % i] * (MOD / i) % MOD;
            finv[i] = finv[i - 1] * inv[i] % MOD;
        }
    }

    long modComb(int n, int r){ // return nCr (先にinitCOMB()必要)
        if(n < r || n < 0 || r < 0) return 0;
        return fac[n] * finv[r] % MOD * finv[n - r] % MOD;
    }

    long comb(long n, long r){ // return nCr
        long num = 1;
        for(long i = 1; i <= r; i++){
            num = num * (n - i + 1) / i;
        }
        return num;
    }

    boolean isPrime(long a){ // aの素数判定
        if(a <= 1) return false;
        for(int i = 2; i * i <= a; i++){
            if(a % i == 0) return false;
        }
        return true;
    }

    int lowerBound(long[] a, long v){ // return 配列a内のv以上の要素の内最低の要素のイテレータ
        int r = a.length;
        int l = -1;
        while(r - l > 1){
            int mid = (r + l) / 2;
            if(a[mid] >= v){
                r = mid;
            }else{
                l = mid;
            }
        }
        return r;
    }

    int upperBound(long[] a, long v){ // return 配列a内のvより大きい要素の内最低の要素のイテレータ
        int r = a.length;
        int l = -1;
        while(r - l > 1){
            int mid = (r + l) / 2;
            if(a[mid] > v){
                r = mid;
            }else{
                l = mid;
            }
        }
        return r;
    }

    String nextPermutation(String s){ // return sの次の順列
        ArrayList<Character> list = new ArrayList<>();
        for(int i = 0; i < s.length(); i++) list.add(s.charAt(i));

        int pivotPos = -1;
        char pivot = 0;
        for(int i = list.size() - 2; i >= 0; i--){
            if(list.get(i) < list.get(i+1)){
    			pivotPos = i;
    			pivot = list.get(i);
    			break;
    		}
    	}

        if(pivotPos == -1 && pivot == 0) return null;

        int L = pivotPos + 1;
        int R = list.size() - 1;
    	int minPos = -1;
    	char min = Character.MAX_VALUE;
    	for(int i = R; i >= L; i--){
    		if(pivot < list.get(i)){
    			if(list.get(i) < min){
    				min = list.get(i);
    				minPos = i;
    			}
    		}
    	}

    	Collections.swap(list, pivotPos, minPos);
    	Collections.sort(list.subList(L, R + 1));

    	StringBuilder sb = new StringBuilder();
    	for(int i=0; i<list.size(); i++) sb.append(list.get(i));

    	return sb.toString();
    }

    boolean nextPermutation(long[] a){
        for(int i = a.length - 1; i > 0; i--){
            if(a[i - 1] < a[i]){
                int swapIndex = find(a[i - 1], a, i, a.length - 1);
                long temp = a[swapIndex];
                a[swapIndex] = a[i - 1];
                a[i - 1] = temp;
                Arrays.sort(a, i, a.length);
                return true;
            }
        }
        return false;
    }

    int find(long dest, long[] a, int s, int e){
        if(s == e){
            return s;
        }
        int m = (s + e + 1) / 2;
        return a[m] <= dest ? find(dest, a, s, m - 1) : find(dest, a, m, e);
    }

    void elimination(int[][] a, int[] b) {
        int n = a.length;
        double f;
        for(int k = 0; k < n - 1; k++){
            for(int i = k + 1; i < n; i++){
                f = - a[i][k] / a[k][k];
                for(int j = k + 1; j < n; j++){
                    a[i][j] += f * a[k][j];
                }
                b[i] += f * b[k];
            }
            for(int i = n - 1; i >= 0; i--){
                for(int j = i + 1; j < n; j++){
                    b[i] -= a[i][j] * b[j];
                }
                b[i] = b[i] / a[i][i];
            }
        }
   }



   class SegmentTree{

       //------------------------------------------------------------
       // 2 * n - 1 : 木全体のノード数
       // i + n - 1 : 配列のi番目が対応するノードの番号
       // 2 * i + 1, 2 * i + 2 : i番目のノードの子ノードの番号
       // (i - 1) / 2 : i番目のノードの親ノードの番号
       //
       // int n = sc.nextInt();
       // long[] a = new long[n];
       // for(int i = 0; i < n; i++) a[i] = sc.nextLong();
       // SegmentTree st = new SegmentTree(a);
       // int l = sc.nextInt() - 1;
       // int r = sc.nextInt() - 1;
       // System.out.println(st.query(l, r));
       //------------------------------------------------------------

       final static long INF = Long.MAX_VALUE / 2;

       // long e = INF; // 単位元
       long e = 0;
       long func(long a, long b){ // 処理
           // return Math.min(a, b);
           return a + b;
       }

       int n; // 配列の要素数を超える最小の2のべき乗
       long[] node;

       SegmentTree(long[] a){
           init(a);
       }

       void init(long[] a){ // 配列aで初期化
           n = 1;
           while(n < a.length){
               n *= 2;
           }
           node = new long[2 * n - 1];
           Arrays.fill(node, e);
           for(int i = 0; i < a.length; i++){
               node[i + n - 1] = a[i];
           }
           for(int i = n - 2; i >= 0; i--){
               node[i] = func(node[2 * i + 1], node[2 * i + 2]);
           }
       }

       void update(int p, long v){ // 配列のp番目をvに変更し、木全体を更新
           p = p + n - 1;
           node[p] = v;
           while(p > 0){
               p = (p - 1) / 2;
               node[p] = func(node[2 * p + 1], node[2 * p + 2]);
           }
       }

       long query(int a, int b){ // 区間[a, b)についてクエリを処理
           return query(a, b, 0, 0, n);
       }

       long query(int a, int b, int k, int l, int r){
           if(r <= a || b <= l) return e;
           if(a <= l && r <= b) return node[k];
           return func(query(a, b, 2 * k + 1, l, (l + r) / 2), query(a, b, 2 * k + 2, (l + r) / 2, r));
       }

   }



}



class Pair implements Comparable<Pair>{
    long a, b;
    public Pair(long i, long j){
        a = i;
        b = j;
    }

    @Override
    public int compareTo(Pair p){
        if(this.b < p.b) return -1;
        else if(this.b > p.b) return 1;
        else return 0;
    }
}



class FastScanner {
    private final InputStream in = System.in;
    private final byte[] buffer = new byte[1024];
    private int ptr = 0;
    private int buflen = 0;
    private boolean hasNextByte(){
        if(ptr < buflen){
            return true;
        }else{
            ptr = 0;
            try{
                buflen = in.read(buffer);
            }catch(IOException e){
                e.printStackTrace();
            }
            if(buflen <= 0){
                return false;
            }
        }
        return true;
    }
    private int readByte(){
        if(hasNextByte()) return buffer[ptr++];
        else return -1;
    }
    private static boolean isPrintableChar(int c){
        return 33 <= c && c <= 126;
    }
    public boolean hasNext(){
        while(hasNextByte() && !isPrintableChar(buffer[ptr])) ptr++;
        return hasNextByte();
    }
    public String next(){
        if(!hasNext()) throw new NoSuchElementException();
        StringBuilder sb = new StringBuilder();
        int b = readByte();
        while(isPrintableChar(b)){
            sb.appendCodePoint(b);
            b = readByte();
        }
        return sb.toString();
    }
    public long nextLong(){
        if(!hasNext()) throw new NoSuchElementException();
        long n = 0;
        boolean minus = false;
        int b = readByte();
        if(b == '-'){
            minus = true;
            b = readByte();
        }
        if(b < '0' || '9' < b){
            throw new NumberFormatException();
        }
        while(true){
            if('0' <= b && b <= '9'){
                n *= 10;
                n += b - '0';
            }else if(b == -1 || !isPrintableChar(b)){
                return minus ? -n : n;
            }else{
                throw new NumberFormatException();
            }
            b = readByte();
        }
    }
    public int nextInt(){
        long nl = nextLong();
        if (nl < Integer.MIN_VALUE || nl > Integer.MAX_VALUE) throw new NumberFormatException();
        return (int) nl;
    }
    public double nextDouble(){
        return Double.parseDouble(next());
    }
}


Measured memory_bytes: 73728