#include<cstdio>
#include<cstring>
#include<cstdlib>
#include<iostream>
#include<algorithm>
#define N 20
#define ll long long 
using namespace std;
ll n,m[N],a[N],m1,e;
ll read()
{
    ll x=0,f=1; char ch=getchar();
    while(ch<‘0‘||ch>‘9‘){if(ch==‘-‘)f=-1; ch=getchar();}
    while(ch>=‘0‘&&ch<=‘9‘){x=x*10+ch-‘0‘; ch=getchar();}
    return x*f;
}
ll exgcd(ll a,ll b,ll &x,ll &y)
{
    if(b==0)
    {
        x=1,y=0;
        return a;
    }
    ll r=exgcd(b,a%b,x,y),tmp;
    tmp=x,x=y,y=tmp-a/b*y;
    return r;
}
ll crt()
{
    ll a1=a[1],a2,m2,d,c;m1=m[1];
    for(ll i=2;i<=n;++i)
    {
        a2=a[i],m2=m[i];
        c=a2-a1;ll x=0,y=0;
        d=exgcd(m1,m2,x,y);
        if(c%d) return  -1;
        x=x*c/d;
        int mod=m2/d;
        x=(mod+x%mod)%mod;
        a1+=m1*x;m1*=mod;
    }
    return  a1;
}
int main()
{
    while(1)
    {
        n=read(),e=read();
        if(n==0&&e==0) break;
        for(int i=1;i<=n;i++) m[i]=read(),a[i]=m[i]-e;
        printf("%lld\n",crt());
    }
    return 0;
}