import java.io.*
import java.util.*
import java.lang.*
import java.lang.Math.*

class PriQueMax<T>() :
        PriorityQueue<T>(Collections.reverseOrder()) {}

private data class Cake(val type: Int, val cnt: Int): Comparable<Cake> {
    override operator fun compareTo(other: Cake): Int {
        if (this.cnt > other.cnt) return 1
        if (this.cnt < other.cnt) return -1
        return 0
    }
}

fun solve() {
    val (K, T)  = rd.readListInt()
    val C = rd.readIntArray()

    val pq = PriQueMax<Cake>()
    for (i in 0 until T) {
        pq.add(Cake(i, C[i]))
    }

    var ans = 0
    var d = 0
    var cur = pq.poll()!!
    var next: Cake? = null
    var res = ""
    while (d < K) {
        res += cur.type.toString()

        if (d == K-1) break
        next = pq.poll()

        if (next == null || next.type == cur.type) {
            ans++
            if (cur.cnt-1 > 0) {
                pq.add(Cake(cur.type, cur.cnt-1))
            }
        } else {
            if (cur.cnt-1 > 0) {
                pq.add(Cake(cur.type, cur.cnt-1))
            }
            cur = next
        }

        d++
    }
    errPrintln(res)

    println(ans)
}


//val rd = debug.Reader("etc", "code-festival-2016-qualc", "b", "sample-3")
val rd = Reader()


fun main(args: Array<String>) {
    try {
        var exception: Throwable? = null
        val sth = Thread(null, SolveThread(), "solveThread", 128 * 1024 * 1024)
        sth.setUncaughtExceptionHandler { _, e -> exception = e }
        sth.start(); sth.join()
        if (exception != null) throw exception!! }
    catch (e: Exception) { throw e }
    finally { rd.close() }
}
const val MOD = 1_000_000_007L; const val INF = Int.MAX_VALUE/2; const val LINF = Long.MAX_VALUE/2
data class IntPair(val first: Int, val second: Int); data class LongPair(val first: Long, val second: Long)
val errPrintln: (Any) -> Unit = { msg -> System.err.println(msg) }
fun rAssert(exp: Boolean, lazyMsg: () -> String = {""}) {
    if (!exp) { val msg = lazyMsg(); errPrintln("EAssert Error: $msg"); throw Error(msg) }
}
fun tAssert(exp: Boolean, lazyMsg: () -> String = {""}) {
    if (!exp) { val msg = lazyMsg(); errPrintln("TAssert Error: $msg"); Thread.sleep(3000) }
}
class SolveThread : Runnable {
    override fun run() { solve() }
}
class Reader {
    private val br = BufferedReader(InputStreamReader(System.`in`))
    val readString: () -> String = { br.readLine() }
    val readInt: () -> Int = { br.readLine().toInt() }
    val readLong: () -> Long = { br.readLine().toLong() }
    val readIntArray: () -> IntArray = { br.readLine().split(' ').map { it.toInt() }.toIntArray() }
    val readLongArray: () -> LongArray = { br.readLine().split(' ').map { it.toLong() }.toLongArray() }
    val readListInt: () -> List<Int> = { br.readLine().split(' ').map { it.toInt() } }
    val readListLong: () -> List<Long> = { br.readLine().split(' ').map { it.toLong() } }
    val readListIntCol: (Int) -> List<Int> = { N -> (0 until N).map { readInt() } }
    val readListLongCol: (Int) -> List<Long> = { N -> (0 until N).map { readLong() } }
    val close: () -> Unit = { br.close() }
    protected fun finalize() { close() }
}


Measured memory_bytes: 53768