import kotlin.math.max

fun main(args: Array<String>){
    val numberCount = readLine()!!.toInt()
    val numberList = readLine()!!.trim().split(" ").map { it.toInt() }
    val primeList = getPrimeList()
    val numberMap = mutableMapOf<Int, Int>()
    var stattIndex = 0
    var ans = 0
    for(i in primeList.indices) {
        primeList[i].toString().map { it.toString().toInt() }.forEach {
            val key = it
            numberMap[key]?.also { numberMap[key] = it+1 } ?: run { numberMap[key] = 1 }
        }
        while (numberMap.filter { it.value > 0 }.any { !numberList.contains(it.key) }) {
            primeList[stattIndex].toString().map { it.toString().toInt() }.forEach { numberMap[it] = numberMap[it]!!-1 }
            stattIndex++
        }
        if(!numberMap.filter { it.value > 0 }.all { numberList.contains(it.key) }) {
            stattIndex = i + 1
        }
        if(stattIndex <= i) {
            var startNum = 0
            if(stattIndex == 0) {
                startNum = 1
            } else {
                startNum = primeList[stattIndex-1] + 1
            }
            var endNum = 5000000
            if(i < primeList.lastIndex) {
                endNum = primeList[i + 1] - 1
            }
            ans = Math.max(ans, endNum - startNum)
        }
    }
    if(ans == 0) {
        ans = -1
    }
    println(ans)
}


fun getPrimeList():List<Int> {
    val max = 5000000
    val primeList = mutableListOf<Int>()
    val flags = arrayOfNulls<Boolean>(max + 1)
    for(i in 2..max) {
        if(flags[i] != null && flags[i]!!) {
            continue
        }
        val maxIndex = max / i
        for(j in 1..maxIndex) {
            flags[i * j] = true
        }
        primeList.add(i)
    }
    return primeList
}