1 /* */
 2 # include <iostream>
 3 # include <stdio.h>
 4 # include <string.h>
 5 # include <string>
 6 # include <cstdlib>
 7 # include <cmath>
 8 # include <climits>
 9 # include <cctype>
10 # include <iomanip>
11 # include <cassert>
12 # include <deque>
13 # include <queue>
14 # include <stack>
15 # include <vector>
16 # include <utility>
17 # include <list>
18 # include <set>
19 # include <map>
20 # include <functional>
21 # include <ctime>
22 # include <algorithm>
23 using namespace std;
24 typedef long long ll;
25 const double pi=acos(-1.0);
26 const int maxn=1e6;
27 const ll mod=1e9+7;
28 # define lson l,m,rt<<1
29 # define rson m+1,r,rt<<1|1
30 # define lowbit(x)(x&(-x))
31 # define lcm(a,b)(a*b/__gcd(a,b))
32
33 int main()
34 {
35     priority_queue<ll, vector<ll>, greater<ll> >q;
36     ll a, x, k=1, ans=0;
37     cin>>a;
38     for( ll i=1; i<=a; i++ )
39     {
40         cin>>x;
41         q.push(x);
42     }
43
44     while( !q.empty())
45     {
46         ll t=q.top();
47         q.pop();
48         if( !q.empty())
49         {
50             ll g=q.top();
51             if( t==g )
52             {
53                 t = t+g;
54                 q.pop();
55                 q.push(t);
56             }
57             else
58             {
59                 t=2*t;
60                 q.push(t);
61                 ans++;
62             }
63         }
64         else
65             break;
66         if( ans>4e5 )///加了这么多还没有合成一个说明不能合成一个了
67         {
68             k=0;
69             break;
70         }
71     }
72     if( k==1 )
73         cout<<ans<<endl;
74     else
75         cout<<-1<<endl;
76 }