#![allow(unused_imports)]
#![allow(non_snake_case, unused)]

use std::cmp::*;
use std::collections::*;
use std::ops::*;

// https://atcoder.jp/contests/hokudai-hitachi2019-1/submissions/10518254 より

macro_rules! eprint {
    ($($t:tt)*) => {{
        use ::std::io::Write;
        let _ = write!(::std::io::stderr(), $($t)*);
    }};
}
macro_rules! eprintln {
    () => { eprintln!(""); };
    ($($t:tt)*) => {{
        use ::std::io::Write;
        let _ = writeln!(::std::io::stderr(), $($t)*);
    }};
}
macro_rules! dbg {
    ($v:expr) => {{
        let val = $v;
        eprintln!("[{}:{}] {} = {:?}", file!(), line!(), stringify!($v), val);
        val
    }}
}

macro_rules! mat {
    ($($e:expr),*) => { Vec::from(vec![$($e),*]) };
    ($($e:expr,)*) => { Vec::from(vec![$($e),*]) };
    ($e:expr; $d:expr) => { Vec::from(vec![$e; $d]) };
    ($e:expr; $d:expr $(; $ds:expr)+) => { Vec::from(vec![mat![$e $(; $ds)*]; $d]) };
}

macro_rules! ok {
    ($a:ident$([$i:expr])*.$f:ident()$(@$t:ident)*) => {
        $a$([$i])*.$f($($t),*)
    };
    ($a:ident$([$i:expr])*.$f:ident($e:expr$(,$es:expr)*)$(@$t:ident)*) => { {
        let t = $e;
        ok!($a$([$i])*.$f($($es),*)$(@$t)*@t)
    } };
}

pub fn readln() -> String {
    let mut line = String::new();
    ::std::io::stdin().read_line(&mut line).unwrap_or_else(|e| panic!("{}", e));
    line
}

macro_rules! read {
    ($($t:tt),*; $n:expr) => {{
        let stdin = ::std::io::stdin();
        let ret = ::std::io::BufRead::lines(stdin.lock()).take($n).map(|line| {
            let line = line.unwrap();
            let mut it = line.split_whitespace();
            _read!(it; $($t),*)
        }).collect::<Vec<_>>();
        ret
    }};
    ($($t:tt),*) => {{
        let line = readln();
        let mut it = line.split_whitespace();
        _read!(it; $($t),*)
    }};
}

macro_rules! _read {
    ($it:ident; [char]) => {
        _read!($it; String).chars().collect::<Vec<_>>()
    };
    ($it:ident; [u8]) => {
        Vec::from(_read!($it; String).into_bytes())
    };
    ($it:ident; usize1) => {
        $it.next().unwrap_or_else(|| panic!("input mismatch")).parse::<usize>().unwrap_or_else(|e| panic!("{}", e)) - 1
    };
    ($it:ident; [usize1]) => {
        $it.map(|s| s.parse::<usize>().unwrap_or_else(|e| panic!("{}", e)) - 1).collect::<Vec<_>>()
    };
    ($it:ident; [$t:ty]) => {
        $it.map(|s| s.parse::<$t>().unwrap_or_else(|e| panic!("{}", e))).collect::<Vec<_>>()
    };
    ($it:ident; $t:ty) => {
        $it.next().unwrap_or_else(|| panic!("input mismatch")).parse::<$t>().unwrap_or_else(|e| panic!("{}", e))
    };
    ($it:ident; $($t:tt),+) => {
        ($(_read!($it; $t)),*)
    };
}


pub fn main() {
    let _ = ::std::thread::Builder::new().name("run".to_string()).stack_size(32 * 1024 * 1024).spawn(run).unwrap().join();
}

const MOD: usize = 998244353;
// const MOD: usize = 1_000_000_007;
// const MOD: i128 = std::i128::MAX/2;
const INF: i64 = std::i64::MAX/2;

pub struct UnionFind {
    data: Vec<i32>,
}

impl UnionFind {
    pub fn new(n: usize) -> Self{
        UnionFind {
            data: vec![-1;n]
        }
    }
    pub fn root(&mut self, x: usize) -> usize{
        if self.data[x]<0 {
            x
        }
        else {
            let p = self.data[x] as usize;
            self.data[x] = self.root(p) as i32;
            self.data[x] as usize
        }
    }
    pub fn unite(&mut self, x: usize, y: usize){
        let rx = self.root(x);
        let ry = self.root(y);
        if rx!=ry {
            let (rx,ry) = if self.data[rx]<=self.data[ry] {
                (rx,ry)
            }
            else{
                (ry,rx)
            };
            self.data[rx] += self.data[ry];
            self.data[ry] = rx as i32;
        }
    }
    pub fn same(&mut self, x: usize, y: usize) ->  bool{
        self.root(x) == self.root(y)
    }
    pub fn size(&mut self, x: usize) -> i32 {
        let r = self.root(x);
        - self.data[r]
    }
}

fn solve() {
    let (n,m) = read!(usize,usize);
    let ab = read!(usize,usize;m);
    let mut uf = UnionFind::new(n);
    for (a,b) in ab {
        uf.unite(a-1,b-1);
    }
    let mut count = 0;
    for i in 0..n {
        if uf.root(i) == i {
            count += 1;
        }
    }
    println!("{}",count-1);
}

fn run() {
    let stack_size = 104_857_600; // 100 MB
    let thd = std::thread::Builder::new().stack_size(stack_size);
    solve();
}


Measured memory_bytes: 3020