//package misc.ukiepc2025

fun main() {
	val (n, m) = readInts()
	val heights = readInts()
	val edges = List(m) {
		val (u, v) = readInts().map { it - 1 }
		Edge(u, v, maxOf(heights[u], heights[v]))
	}.sortedBy { it.hMax }
	var ans = Int.MAX_VALUE
	for (hLow in heights.sorted()) {
		if (hLow > heights[0] || hLow > heights[1]) break
		val dsu = DisjointSetUnion(n)
		for (e in edges) {
			val (u, v, hMax) = e
			if (heights[u] < hLow || heights[v] < hLow) continue
			if (dsu[u] == dsu[v]) continue
			dsu.unite(u, v)
			if (dsu[0] == dsu[1]) {
				ans = minOf(ans, hMax - hLow)
				break
			}
		}
	}
	println(ans)
}

data class Edge(val u: Int, val v: Int, val hMax: Int)

class DisjointSetUnion(n: Int) {
	internal val p: IntArray = IntArray(n) { it }
	internal val r = java.util.Random(566)
	internal operator fun get(v: Int): Int = if (p[v] == v) v else { p[v] = get(p[v]); p[v] }
	internal fun unite(v: Int, u: Int) = if (r.nextBoolean()) p[get(v)] = get(u) else p[get(u)] = get(v)
}

private fun readStrings() = readln().split(" ")
private fun readInts() = readStrings().map { it.toInt() }