#include <bits/stdc++.h>
#define int unsigned long long
using namespace std;

int n,ans=0;
int a[100005],b[100005],c[100005];

int p[100005][205];

signed main()
{
	cin>>n;
	n++;
	for (int i=1;i<=n;i++)  cin>>a[i];
	
	b[1]=1,c[n]=a[n];
	for (int i=2;i<=n;i++)  b[i]=(b[i-1]-a[i-1])*2;
	for (int i=n-1;i>=1;i--)  c[i]=c[i+1]+a[i];
	for (int i=1;i<=n;i++)  ans+=min(b[i],c[i]);
	for (int i=1;i<=n;i++)
	{
		if (a[i]>b[i])  return cout<<-1<<endl,0;
	}
	cout<<ans<<endl;
	
	return 0;
}
