/**
 * 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; }


#ifndef ATCODER_DSU_HPP
#define ATCODER_DSU_HPP 1

#include <algorithm>
#include <cassert>
#include <vector>

namespace atcoder {

// Implement (union by size) + (path compression)
// Reference:
// Zvi Galil and Giuseppe F. Italiano,
// Data structures and algorithms for disjoint set union problems
struct dsu {
  public:
    dsu() : _n(0) {}
    dsu(int n) : _n(n), parent_or_size(n, -1) {}

    int merge(int a, int b) {
        assert(0 <= a && a < _n);
        assert(0 <= b && b < _n);
        int x = leader(a), y = leader(b);
        if (x == y) return x;
        if (-parent_or_size[x] < -parent_or_size[y]) std::swap(x, y);
        parent_or_size[x] += parent_or_size[y];
        parent_or_size[y] = x;
        return x;
    }

    bool same(int a, int b) {
        assert(0 <= a && a < _n);
        assert(0 <= b && b < _n);
        return leader(a) == leader(b);
    }

    int leader(int a) {
        assert(0 <= a && a < _n);
        if (parent_or_size[a] < 0) return a;
        return parent_or_size[a] = leader(parent_or_size[a]);
    }

    int size(int a) {
        assert(0 <= a && a < _n);
        return -parent_or_size[leader(a)];
    }

    std::vector<std::vector<int>> groups() {
        std::vector<int> leader_buf(_n), group_size(_n);
        for (int i = 0; i < _n; i++) {
            leader_buf[i] = leader(i);
            group_size[leader_buf[i]]++;
        }
        std::vector<std::vector<int>> result(_n);
        for (int i = 0; i < _n; i++) {
            result[i].reserve(group_size[i]);
        }
        for (int i = 0; i < _n; i++) {
            result[leader_buf[i]].push_back(i);
        }
        result.erase(
            std::remove_if(result.begin(), result.end(),
                           [&](const std::vector<int>& v) { return v.empty(); }),
            result.end());
        return result;
    }

  private:
    int _n;
    // root node: -1 * component size
    // otherwise: parent
    std::vector<int> parent_or_size;
};

}  // namespace atcoder

#endif  // ATCODER_DSU_HPP

void ssolve(std::istream& in, std::ostream& out){
    int n, m; s2(n, m);
    atcoder::dsu uf(n);
    rep(i, 0, m){
        int a, b; s2(a, b);
        uf.merge(a - 1, b - 1);
    }
    set<int> leaders;
    for(int i = 0; i < n; i++){
        leaders.insert(uf.leader(i));
    }
    p(leaders.size() - 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: 7856