#pragma GCC optimize ("Ofast")
#include<bits/stdc++.h>
using namespace std;
inline int my_getchar_unlocked(){
  static char buf[1048576];
  static int s = 1048576;
  static int e = 1048576;
  if(s == e && e == 1048576){
    e = fread_unlocked(buf, 1, 1048576, stdin);
    s = 0;
  }
  if(s == e){
    return EOF;
  }
  return buf[s++];
}
inline void rd(int &x){
  int k;
  int m=0;
  x=0;
  for(;;){
    k = my_getchar_unlocked();
    if(k=='-'){
      m=1;
      break;
    }
    if('0'<=k&&k<='9'){
      x=k-'0';
      break;
    }
  }
  for(;;){
    k = my_getchar_unlocked();
    if(k<'0'||k>'9'){
      break;
    }
    x=x*10+k-'0';
  }
  if(m){
    x=-x;
  }
}
inline int rd_int(void){
  int x;
  rd(x);
  return x;
}
struct MY_WRITER{
  char buf[1048576];
  int s;
  int e;
  MY_WRITER(){
    s = 0;
    e = 1048576;
  }
  ~MY_WRITER(){
    if(s){
      fwrite_unlocked(buf, 1, s, stdout);
    }
  }
}
;
MY_WRITER MY_WRITER_VAR;
void my_putchar_unlocked(int a){
  if(MY_WRITER_VAR.s == MY_WRITER_VAR.e){
    fwrite_unlocked(MY_WRITER_VAR.buf, 1, MY_WRITER_VAR.s, stdout);
    MY_WRITER_VAR.s = 0;
  }
  MY_WRITER_VAR.buf[MY_WRITER_VAR.s++] = a;
}
inline void wt_L(char a){
  my_putchar_unlocked(a);
}
inline void wt_L(int x){
  int s=0;
  int m=0;
  char f[10];
  if(x<0){
    m=1;
    x=-x;
  }
  while(x){
    f[s++]=x%10;
    x/=10;
  }
  if(!s){
    f[s++]=0;
  }
  if(m){
    my_putchar_unlocked('-');
  }
  while(s--){
    my_putchar_unlocked(f[s]+'0');
  }
}
int cnt[12];
int arr[7] = {0,2,4,5,7,9,11};
int dame[12];
int main(){
  int Lj4PdHRW, i;
  int res = -1;
  int fg = 0;
  int KL2GvlyY = rd_int();
  for(Lj4PdHRW=(0);Lj4PdHRW<(KL2GvlyY);Lj4PdHRW++){
    cnt[rd_int()] = 1;
  }
  for(i=(0);i<(12);i++){
    dame[i] = 1;
  }
  for(i=(0);i<(7);i++){
    dame[arr[i]] = 0;
  }
  for(i=(0);i<(12);i++){
    int j;
    for(j=(0);j<(12);j++){
      if(cnt[(i+j)%12] && dame[j]){
        goto WYIGIcGE;
      }
    }
    res = i;
    fg++;
    WYIGIcGE:;
  }
  if(fg > 1){
    res = -1;
  }
  wt_L(res);
  wt_L('\n');
  return 0;
}
// cLay varsion 20200509-1

// --- original code ---
// int cnt[12];
// int arr[7] = {0,2,4,5,7,9,11}, dame[12];
// {
//   int res = -1, fg = 0;
//   REP(rd_int()) cnt[rd_int()] = 1;
//   rep(i,12) dame[i] = 1;
//   rep(i,7) dame[arr[i]] = 0;
//   rep(i,12){
//     rep(j,12) if(cnt[(i+j)%12] && dame[j]) break_continue;
//     res = i;
//     fg++;
//   }
//   if(fg > 1) res = -1;
//   wt(res);
// }