struct Scanner {
    private var elements = [String]()
    private var index = 0

    mutating func peek() -> String {
        while elements.count == index {
            elements = readLine()!.split(separator: " ").map(String.init)
            index = 0
        }
        return elements[index]
    }

    mutating func next() -> String {
        let value = peek()
        index += 1
        return value
    }

    mutating func nextInt() -> Int {
        return Int(next())!
    }

    mutating func nextDouble() -> Double {
        return Double(next())!
    }
}

let MOD = 998244353

struct Zn: CustomStringConvertible {
    let n: Int

    init(_ n: Int) {
        self.n = n
    }

    var description: String {
        String(self.n)
    }

    static var zero = Zn(0)
    static var one = Zn(1)

    static func + (lhs: Self, rhs: Self) -> Self {
        let n = lhs.n + rhs.n
        return Zn(n < MOD ? n : n - MOD)
    }

    static func + (lhs: Self, rhs: Int) -> Self {
        return lhs + Zn(rhs)
    }

    static func + (lhs: Int, rhs: Self) -> Self {
        return Zn(lhs) + rhs
    }

    static func - (lhs: Self, rhs: Self) -> Self {
        let n = lhs.n - rhs.n
        return Zn(n >= 0 ? n : n + MOD)
    }

    static func - (lhs: Self, rhs: Int) -> Self {
        return lhs - Zn(rhs)
    }

    static func - (lhs: Int, rhs: Self) -> Self {
        return Zn(lhs) - rhs
    }

    static func * (lhs: Self, rhs: Self) -> Self {
        Zn((lhs.n * rhs.n) % MOD)
    }

    static func * (lhs: Self, rhs: Int) -> Self {
        return lhs * Zn(rhs)
    }

    static func * (lhs: Int, rhs: Self) -> Self {
        return Zn(lhs) * rhs
    }

    static func += (lhs: inout Self, rhs: Self) {
        lhs = lhs + rhs
    }

    static func += (lhs: inout Self, rhs: Int) {
        lhs = lhs + Zn(rhs)
    }

    static func -= (lhs: inout Self, rhs: Self) {
        lhs = lhs - rhs
    }

    static func -= (lhs: inout Self, rhs: Int) {
        lhs = lhs - Zn(rhs)
    }

    static func *= (lhs: inout Self, rhs: Self) {
        lhs = lhs * rhs
    }

    static func *= (lhs: inout Self, rhs: Int) {
        lhs = lhs * Zn(rhs)
    }

    func pow(_ n: Int) -> Self {
        if n == 0 {
            return Zn(1)
        } else if n % 2 == 1 {
            return self * self.pow(n - 1)
        } else {
            return (self * self).pow(n / 2)
        }
    }

    var inverse: Self {
        self.pow(MOD - 2)
    }
}

var scanner = Scanner()
let N = scanner.nextInt()
let M = scanner.nextInt()

var G = [[Int]](repeating: [], count: N)

for _ in 0 ..< M {
    let u = scanner.nextInt() - 1
    let v = scanner.nextInt() - 1
    G[u].append(v)
    G[v].append(u)
}

var visited = [Bool](repeating: false, count: N)

func dfs(_ v: Int) {
    visited[v] = true
    for c in G[v] where !visited[c] {
        dfs(c)
    }
}

var answer = 0
for i in 0 ..< N where !visited[i] {
    answer += 1
    dfs(i)
}
print(answer - 1)
