import java.io.IOException
import java.lang.NumberFormatException
import java.lang.StringBuilder
import kotlin.math.absoluteValue
import kotlin.math.min

object CMain{
    fun solve(){
        val n = nextInt()
        val m = nextInt()
        val dsu = Dsu(n)
        for(i in 1..m){
            val a = nextInt() - 1
            val b = nextInt() - 1
            dsu.merge(a, b)
        }

        var ans = 0
        for (i in 0 until n){
            if(i == dsu.leader(i)) ans ++
        }

        println(ans - 1)

    }





    // Scanner based on uwi-san's Java code
    val stream = System.`in`
    val buffer = ByteArray(1024)
    var ptr = 0
    var buflen = 0

    fun hasNextByte(): Boolean{
        if(ptr < buflen){
            return true
        }else{
            ptr = 0
            try{
                buflen = stream.read(buffer)
            }catch (e: IOException){
                e.printStackTrace()
            }
            if(buflen <= 0){
                return false
            }
        }
        return true
    }

    fun readByte() =
        if(hasNextByte()) buffer[ptr++]
        else -1

    fun isPrintableChar(c: Byte) = c in 33..126

    fun hasNext(): Boolean{
        while(hasNextByte() && !isPrintableChar(buffer[ptr])) ptr++
        return hasNextByte()
    }

    fun next(): String{
        if(!hasNext()) throw NoSuchElementException()
        val sb = StringBuilder()
        var b = readByte()
        while(isPrintableChar(b)){
            sb.appendCodePoint(b.toInt())
            b = readByte()
        }
        return sb.toString()
    }

    fun nextLong(): Long{
        if(!hasNext()) throw NoSuchElementException()
        var n = 0L
        var minus = false
        var b = readByte()
        if(b == '-'.toByte()){
            minus = true
            b = readByte()
        }
        if(b !in '0'.toByte()..'9'.toByte())
            throw NumberFormatException()
        while(true){
            if(b in '0'.toByte()..'9'.toByte()){
                n *= 10
                n += b - '0'.toByte()
            }else if(!isPrintableChar(b)){
                return if(minus) -n else n
            }else{
                throw NumberFormatException()
            }
            b = readByte()
        }
    }

    fun nextInt(): Int{
        val n1 = nextLong()
        if(n1 !in Int.MIN_VALUE..Int.MAX_VALUE)
            throw NumberFormatException()
        return n1.toInt()
    }

    fun NextDouble() = next().toDouble()
}

class Dsu(val n: Int = 0) {
    // root node: -1 * component size
    // otherwise: parent
    var parentOrSize = IntArray(n) { -1 }

    fun merge(a: Int, b: Int): Int {
        assert(a in 0 until n)
        assert(b in 0 until n)
        var x = leader(a)
        var y = leader(b)
        if (x == y) return x
        if (-parentOrSize[x] < -parentOrSize[y]) {
            var tmp = x
            x = y
            y = tmp
        }
        parentOrSize[x] += parentOrSize[y]
        parentOrSize[y] = x
        return x
    }

    fun same(a: Int, b: Int): Boolean {
        assert(a in 0 until n)
        assert(b in 0 until n)
        return leader(a) == leader(b)
    }

    fun leader(a: Int): Int {
        assert(a in 0 until n)
        if (parentOrSize[a] < 0) return a
        parentOrSize[a] = leader(parentOrSize[a])
        return parentOrSize[a]
    }

    fun size(a: Int): Int {
        assert(a in 0 until n)
        return -parentOrSize[leader(a)]
    }
}

fun main(){
    CMain.solve()
}



Measured memory_bytes: 35084