import std.conv, std.functional, std.range, std.stdio, std.string;
import std.algorithm, std.array, std.bigint, std.bitmanip, std.complex, std.container, std.math, std.mathspecial, std.numeric, std.regex, std.typecons;
import core.bitop;

class EOFException : Throwable { this() { super("EOF"); } }
string[] tokens;
string readToken() { for (; tokens.empty; ) { if (stdin.eof) { throw new EOFException; } tokens = readln.split; } auto token = tokens.front; tokens.popFront; return token; }
int readInt() { return readToken.to!int; }
long readLong() { return readToken.to!long; }
real readReal() { return readToken.to!real; }

bool chmin(T)(ref T t, in T f) { if (t > f) { t = f; return true; } else { return false; } }
bool chmax(T)(ref T t, in T f) { if (t < f) { t = f; return true; } else { return false; } }

int binarySearch(alias pred, T)(in T[] as) { int lo = -1, hi = cast(int)(as.length); for (; lo + 1 < hi; ) { const mid = (lo + hi) >> 1; (unaryFun!pred(as[mid]) ? hi : lo) = mid; } return hi; }
int lowerBound(T)(in T[] as, T val) { return as.binarySearch!(a => (a >= val)); }
int upperBound(T)(in T[] as, T val) { return as.binarySearch!(a => (a > val)); }




void main() {
  try {
    for (; ; ) {
      const N = readInt();
      auto A = new long[N + 1];
      foreach (d; 0 .. N + 1) {
        A[d] = readLong();
      }
      
      auto mns = new long[N + 1];
      auto mxs = new long[N + 1];
      foreach_reverse (d; 0 .. N) {
        // b[d] + 2 c[d] = A[d + 1] + b[d + 1] + c[d + 1]
        mns[d] = (A[d + 1] + mns[d + 1] + 1) / 2;
        mxs[d] = A[d + 1] + mxs[d + 1];
      }
      debug {
        writeln("mns = ", mns);
        writeln("mxs = ", mxs);
      }
      
      long now = 1 - A[0];
      if (mns[0] <= now && now <= mxs[0]) {
        long ans;
        foreach (d; 0 .. N + 1) {
          ans += A[d] + now;
          if (d == N) {
            break;
          }
          now *= 2;
          now -= A[d + 1];
          chmin(now, mxs[d + 1]);
        }
        writeln(ans);
      } else {
        writeln(-1);
      }
    }
  } catch (EOFException e) {
  }
}
