C++ :

#include <cstdio>
#include <cstring>

using namespace std;

long long high[1000],f[1000];
int n;

int main(){
	scanf("%d",&n);
	memset(f,50,sizeof(f));
	for(int i=1;i<=n;i++){
		scanf("%lld",&high[i]);
	}
	f[1]=0;
	for(int i=2;i<=n;i++){
		if(high[i]==high[i-1]+1){
			f[i]=f[i-1]+1;
		}
		//从哪里退
		for(int j=3;j<=i-1;j++){
			if(f[j]>50000000) continue;
			//枚举退到哪里
			for(int k=1;k<=j-2;k++){
				long long tmp=f[j]+j-k+1;
				long long kf=(1LL)<<(j-k);
				if(high[k]+kf>=high[i]&&tmp<f[i]){
					f[i]=tmp;
				} 
			} 
		} 
	}
	if(f[n]>50000000){
		printf("-1");
	}
	else{
		printf("%lld",f[n]);
	}
	fclose(stdin);
	fclose(stdout);
	return 0;
}