#include<bits/stdc++.h>
using namespace std;
#define int long long
const int N=2e2+10;
const int inf=1e18;
int dp[N];
int n;
int d[N];
signed main(){
	for (int i=0;i<=N-1;i++) dp[i]=inf;
	cin>>n;
	for (int i=1;i<=n;i++) cin>>d[i];
	dp[1]=0;
	for (int i=2;i<=n;i++){
		if (d[i]-d[i-1]==1 && dp[i-1]!=inf) dp[i]=dp[i-1]+1;
		for (int j=1;j<=i-1;j++){
			int high=dp[i-j];
			int maxn=high+(1<<j);
			for (int k=i-j+1;k<=n && d[k]<=maxn;k++){
				dp[k]=min(dp[k],dp[i]+j+1);
			}
		}
	}
	if (dp[n]!=inf) cout<<dp[n];
	else cout<<-1;
	return 0;
}