#include<bits/stdc++.h>
using namespace std ;
typedef long long ll ;
#define el '\n'
const ll maxn = 1e5 + 100 ;
const ll maxk = 19 ;
const ll mod = 1e9 + 7 ;
const ll oo = 1e15 + 7 ;
ll n , a[maxn] , dp[maxn][maxk][maxk] , dp2[maxn] ;
void solve(){
ll mx = 0 ;
cin>>n ; for(ll i = 0 ; i < n ; i++){cin>>a[i];a[i] = max(mx,a[i]);}
if(maxk <= mx){cout<<-1<<el;return;}
for(ll i = 0 ; i < n ; i++){
for(ll k = 0 ; k < maxk ; k++){
dp[i][0][k] = oo ;
}
}
for(ll i = 0 ; i < n ; i++){
dp[i][0][a[i]] = 0 ;
if(0 < a[i]){dp[i][0][a[i]-1] = 1;}
}
for(ll x = 1 ; x < maxk-1 ; x++){
for(ll i = 0 ; i < n ; i++){
if(n < i+(1<<x)){break;}
for(ll k = 0 ; k < maxk-1 ; k++){
ll A1 = dp[i][x-1][k+1] + dp[i+(1<<(x-1))][x-1][k+1] + 1 ;
ll A2 = dp[i][x-1][k] + dp[i+(1<<(x-1))][x-1][k] ;
ll A = min(A1,A2) ; A = min(A,oo) ;
dp[i][x][k] = A ;
}
}
}
for(ll i = 0 ; i < n; i++){
dp2[i] = oo ;
for(ll x = 0 ; x < maxk-1 ; x++){
ll ind = i-(1<<x) ;
if(i-(1<<x) == -1){dp2[i] = min(dp2[i],dp[0][x][0]);}
if(ind < 0){continue;}
dp2[i] = min(dp2[i],dp2[ind]+dp[ind+1][x][0]) ;
}
}
cout<<((dp2[n-1] < oo) ? dp2[n-1] : -1)<<el ;
return ;
}
int main(){
solve() ;
}
