import sequtils,strutils

proc ctoi(c : char):int8=
    return c.int8 - '0'.int8


var
    sosuu : seq[int]
    j : int
    t : int8
    N = stdin.readline.parseInt
    A = stdin.readline.split.map(parseInt)
    aset : set[int8]
    flag : array[5_000_001,bool]
    flag2 : bool
    flag3 : bool
    Mlen = 0
    li : int
sosuu = @[0]
for i in 2..5_000_000:
    if flag[i] == false:
        j = i * 2
        sosuu.add(i)
        while j < 5_000_000:
            flag[j] = true
            j += i
sosuu.add(5_000_001)
for a in A:
    aset.incl(a.int8)
    
flag2 = false
var bset = aset
for i,n in sosuu[1..<sosuu.high]:
    flag3 = false
    for s in $n:
        t = s.ctoi
        if t in aset and flag2:
            bset = bset - {t}
        elif t in aset:
            discard
        else:
            flag3 = true
            flag2 = false
            break
    if flag2 == false and flag3 == false:
        bset = aset
        for s in $n:
            t = s.ctoi
            bset = bset - {t}
        flag2 = true
        li = i + 1
    if flag2 == true and bset.card == 0:
        Mlen = max(Mlen,sosuu[i + 2] - sosuu[li - 1] - 2)
        #echo sosuu[i + 2]," ",sosuu[li - 1]
if Mlen > 0:
    echo Mlen
else:
    echo -1