/**
 * code generated by JHelper
 * More info: https://github.com/AlexeyDmitriev/JHelper
 * @author
 */

#include <iostream>
#include <fstream>

#include <iostream>
#include "bits/stdc++.h"

using namespace std;
typedef unsigned long long ull;
typedef long long ll;
typedef int                     integer;
#define int                     long long
typedef pair<int, int>          pii;
#define vii                     vector<pair<int,int>>
#define vi                      vector<int>
#define vvi                     vector<vector<int>>
#define rep(i, a, b)            for(int i = a; i < (b); i++)
#define s(x)                    in>>x
#define s2(x,y)                 in>>x>>y
#define s3(x,y,z)               in>>x>>y>>z
#define s4(x,y,z,w)             in>>x>>y>>z>>w
#define p(x)                    out<<(x)<<endl
#define p2(x,y)                 out<<(x)<<' '<<(y)<<endl
#define p3(x,y,z)               out<<(x)<<' '<<(y)<<' '<<(z)<<endl
#define p4(x,y,z,w)             out<<(x)<<' '<<(y)<<' '<<(z)<<' '<<(w)<<endl
#define p1d(a,n)                for(int ix=0;ix<n-1;ix++) out<<a[ix]<<' '; out<<a[n-1]<<'\n';
#define p2d(a,n,m)              for(int ix=0;ix<n;ix++){ for(int jx=0;jx<m;jx++) out<<a[ix][jx]<<" "; out<<endl;}
#define pl(x)                   out<<(x)
#define pll(x)                  out<<(x)<<' '
#define range(v)                v.begin(),v.end()
#define all(x)                  begin(x), end(x)
#define sz(x)                   (int)(x).size()
#define rst(x)                  memset(x, -1, sizeof(x))
#define UNVISITED               -1
#define VISITED                 0
#define EXPLORED                2

#define EACH(x, a) for (auto& x: a)
template<class T> void read_v(T& x, std::istream& in) {
    in >> (x);
}
template<class A> void read_vec(vector<A>& x, std::istream& in) {
    EACH(a, x)
        read_v(a, in);
}

ll iceildiv(const ll a, const ll b){ if(a==0){return 0;} return (1 + ((a - 1)/b));}
bool sortpairbysec(const pair<int,int> &a,const pair<int,int> &b){return (a.second < b.second);}
template <typename T> T factmod(T a, T modulus){ int res = 1, i; for (i = 2; i <= a; i++){ res *= i; res%=modulus;} return res; }
template <typename T> int powmod(T a, T b, const int m){ int ans=1; while(b>0){if(b&1){ans*=a;ans%=m;}a*=a;a%=m;b>>=1;} return ans; }
template <typename T> int power(T a, T b){ int ans=1; while(b>0){if(b&1){ans*=a;}a*=a;b>>=1;}return ans;}
template <typename T> void setmin(T& a, T b) { if (b < a) a = b; }
template <typename T> void setmax(T& a, T b) { if (b > a) a = b; }


/**
 * Author: Lukas Polacek
 * Date: 2009-10-26
 * License: CC0
 * Source: folklore
 * Description: Disjoint-set data structure.
 * Time: $O(\alpha(N))$
 */


struct UF {
	vi e;
	UF(int n) : e(n, -1) {}
	bool sameSet(int a, int b) { return find(a) == find(b); }
	int size(int x) { return -e[find(x)]; }
	int find(int x) { return e[x] < 0 ? x : e[x] = find(e[x]); }
	bool join(int a, int b) {
		a = find(a), b = find(b);
		if (a == b) return false;
		if (e[a] > e[b]) swap(a, b);
		e[a] += e[b]; e[b] = a;
		return true;
	}
};

void ssolve(std::istream& in, std::ostream& out){
    int n, m; s2(n, m);
    UF uf(n);
    rep(i, 0, m){
        int a, b; s2(a, b);
        uf.join(a - 1, b - 1);
    }
    int ans = 0;
    rep(i, 0, n){
        if(uf.find(i) == i){ // count leaders
            ans++;
        }
    }
    p(ans - 1);
    return;
}

class CConnectCities {
public:
void solve(std::istream& in, std::ostream& out) {
    ios_base::sync_with_stdio(false);
    in.tie(nullptr);out.tie(nullptr);
    int t;
    int READ = 0;
    if(READ) s(t);
    else t = 1;
    while(t--){
        ssolve(in, out);
    }
}
};

integer main() {
    CConnectCities solver;
    std::istream& in(std::cin);
     std::ostream& out(std::cout);
     solver.solve(in, out);
}

Measured memory_bytes: 3992