/* 4317 */

 #include <iostream>

 #include <sstream>

 #include <string>

 #include <map>

 #include <queue>

 #include <set>

 #include <stack>

 #include <vector>

 #include <deque>

 #include <algorithm>

 #include <cstdio>

 #include <cmath>

 #include <ctime>

 #include <cstring>

 #include <climits>

 #include <cctype>

 #include <cassert>

 #include <functional>

 #include <iterator>

 #include <iomanip>

 using namespace std;

 //#pragma comment(linker,"/STACK:102400000,1024000")

 #define sti                set<int>

 #define stpii            set<pair<int, int> >

 #define mpii            map<int,int>

 #define vi                vector<int>

 #define pii                pair<int,int>

 #define vpii            vector<pair<int,int> >

 #define rep(i, a, n)     for (int i=a;i<n;++i)

 #define per(i, a, n)     for (int i=n-1;i>=a;--i)

 #define clr                clear

 #define pb                 push_back

 #define mp                 make_pair

 #define fir                first

 #define sec                second

 #define all(x)             (x).begin(),(x).end()

 #define SZ(x)             ((int)(x).size())

 #define lson            l, mid, rt<<1

 #define rson            mid+1, r, rt<<1|1

 const int INF = 0x3f3f3f3f;

 int dp[][(<<)+];

 int Bit[];

 int a[];

 int C[(<<)+];

 int n;

 int getBit(int x) {

     int ret = ;

     while (x) {

         ret += (x & );

         x >>= ;

     }

     return ret;

 }

 void Init() {

     int mst = <<;

     rep(i, , mst)

         C[i] = getBit(i);

 }

 void solve() {

     rep(j, , ) {

         Bit[j] = ;

         rep(i, , n) {

             if (a[i] & (<<j))

                 Bit[j] |= (<<i);

         }

     }

     memset(dp, INF, sizeof(dp));

     dp[][] = ;

     int mst = <<n;

     int mask = mst - ;

     int ans, tmp;

     rep(i, , ) {

         rep(j, , mst) {

             if (dp[i][j] == INF)

                 continue;

             int ov = j & Bit[i];

             int one = Bit[i] ^ j;

             int zero = mask & ~one;

             rep(k, , mst) {

                 if (ov & ~k)

                     continue;

                 int other_ov = k & ~ov;

                 if (other_ov & zero)

                     continue;

                 tmp = C[other_ov];

                 int val = one ^ other_ov;

                 if (C[val] & ) {

                     if (val == mask)

                         continue;

                     ++tmp;

                 }

                 tmp <<= i;

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

             }

         }

     }

     ans = dp[][];

     if (ans == INF)

         puts("impossible");

     else

         printf("%d\n", ans);

 }

 int main() {

     ios::sync_with_stdio(false);

     #ifndef ONLINE_JUDGE

         freopen("data.in", "r", stdin);

         freopen("data.out", "w", stdout);

     #endif

     Init();

     while (scanf("%d", &n)!=EOF) {

         rep(i, , n)

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

         solve();

     }

     #ifndef ONLINE_JUDGE

         printf("time = %d.\n", (int)clock());

     #endif

     return ;

 }

 from copy import deepcopy

 from random import randint, shuffle

 import shutil

 import string

 def GenDataIn():

     with open("data.in", "w") as fout:

         t =

         bound = **

         # fout.write("%d\n" % (t))

         for tt in xrange(t):

             n = randint(, )

             fout.write("%d\n" % (n))

             dataList = []

             for i in xrange(n):

                 x = randint(, bound)

                 dataList.append(x)

             fout.write(" ".join(map(str, dataList)) + "\n")

 def MovDataIn():

     desFileName = "F:\eclipse_prj\workspace\hdoj\data.in"

     shutil.copyfile("data.in", desFileName)

 if __name__ == "__main__":

     GenDataIn()

     MovDataIn()