#include<iostream>

 #include<cstdio>

 #include<cstring>

 #include<cmath>

 using namespace std;

 int n,a[],f[]; 

 int main() {

     memset(f,0x3f3f3f,sizeof(f));

     scanf("%d",&n);

     for(int i = ;i <= n; i++)

         scanf("%d",&a[i]);

     f[] = ;

     for(int i = ;i <= n; i++) {

         if(a[i] == a[i-] + ) f[i] = f[i-] + ;

         for(int j = ;j <= i - ; j++)

             for(int k = ;k < i - j; k++)//枚举退到哪里可以跳上去

                 if(a[j] + (int)pow(,k) >= a[i])//可以跳上去了

                     f[i] = min(f[i],f[j+k] + k + );

     }

     if(f[n] >= ) printf("-1");

     else

         printf("%d",f[n]);

     return ;

 }