/*
    ID:qcx97811
    LANG:C++
    PROG:nuggets
*/
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <math.h>

int n;
int num[16];
int idx[16],total_idx;
int can[2000000];
int cmp(const void *a,const void *b)
{//快速排序从小到大模板
    int *c = (int *)a;
    int *d = (int *)b;
    if(*c > *d)
        return 1;
    if(*c == *d)
        return 0;
    return -1;   
}
int gcd(int a,int b)
{//公约数
    int tmp;
    if(a < b)
        {
            tmp = a;
            a = b;
            b =tmp;       
        }
    while(b)
        {
            tmp = a;
            a = b;
            b = tmp%b;
        }
    return a;
}
int main(void)
{
    freopen("nuggets.in","r",stdin);
    freopen("nuggets.out","w",stdout);
    int i,j;
    int k,tmp;
    scanf("%d",&n);
    for(i = 0;i < n;i++)
        {//scanf
            scanf("%d",&num[i]);
        }   
    qsort(num,n,sizeof(num[0]),cmp);//sort the number
    if((1 == num[0]) || (1 == n))
        {//only one num or an one in the num
            printf("0\n");
            return 0;
        }
    else
        {
           memset(idx,0,sizeof(idx));
           memset(can,0,sizeof(can));
           if(2 == n)
             {//2个数的话可以直接算出来
                if(1 == gcd(num[0],num[1]))
                      {//如果互素的话
                           printf("%d\n",num[0]*num[1]-num[0]-num[1]);
                        return 0;
                      }
                else
                    {//不互素 肯定无
                        printf("0\n");
                        return 0;
                    }
             }
           tmp = num[1] - num[0];
           for(i = 2;i < n;i++)
             {//看是否为等差数列
            if(num[i]-num[i-1] != tmp)
               break;
             }
           if(i == n)
             {//如果是等差数列的话 也不可能有
            printf("0\n");
            return 0;
             }
           can[0] = 0;
           total_idx = 0;
           for(i = num[0];i < 2000000;i++ )
             {//下标是为了不超过内存16M 同时最大
                j = 0;
                for(k = 1;k < n;k++)
                   {//增大能表示的数
                      if(num[k]+can[idx[k]]<num[j]+can[idx[j]])
                        j = k;
                   }
                tmp = num[j]+can[idx[j]];
                can[++total_idx] = tmp;
                if(total_idx > num[0]-1 && (can[total_idx]-can[total_idx-num[0]+1] == num[0]-1))
                   {//如果已经找到最大数,因为后面的数都连续了
                    tmp = total_idx-num[0]+1;
                     while(can[tmp] - can[tmp-1] == 1)
                    {//寻找最大的数
                       tmp--; 
                    }    
                     printf("%d\n",can[tmp]-1);//输出最大数
                     return 0;               
                   }
                for(k = 0;k < n;k++)
                    {//改变相应的下标
                        if(num[k]+can[idx[k]] == tmp)
                            idx[k]++;
                    }
               }
//            for(i = 0;i <= total_idx;i++)
//              printf("%d\n",can[i]);
            printf("0\n");       
        }
    return 0;
}