#define M 201
#include <climits>
#include <fstream>
using namespace std;
int power(int x,int y)//求x的y次方
{
	int i,p=1;
	for(i=1;i<=y;i++)p*=x;
	return p;
}

int main()
{
	int i,j,k,n,h[M]={0},f[M]={0},m;
	ifstream fin("ladder.in");
	ofstream fout("ladder.out");
	fin>>n;
	for(i=1;i<=n;i++)	fin>>h[i];
	for(i=2;i<=n;i++)
	{
		j=i-1;
		m=INT_MAX;
		if(h[i]-h[i-1]==1)m=f[i-1]+1;
		for(k=1;k<=j;k++)
		{
			if(h[i]>=h[i-k]+power(2,k))
				m=(f[i-k]+k+1<m?f[i-k]+k+1:m);
		}
		f[i]=m;
	}
	if(f[n]>0)
		fout<<f[n]<<endl;
	else
		fout<<-1<<endl;
	fin.close();fout.close();
	return 0;
}