#include <algorithm>
#include <complex>
#include <cstdlib>
#include <ctime>
#include <time.h>
#include <iomanip>
#include <iostream>
#include <map>
#include <math.h>
#include <queue>
#include <random>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <vector>
#include <numeric>
#include <limits>
#include <type_traits>
#include <locale>
#include <omp.h>

using namespace std;
#define SAY_YES cout << "YES" << endl;
#define SAY_Yes cout << "Yes" << endl;
#define SAY_NO cout << "NO" << endl;
#define SAY_No cout << "No" << endl;
#define IFYES(TRUE_OR_FALSE)   \
    if (TRUE_OR_FALSE)         \
    {                          \
        cout << "YES" << endl; \
    }                          \
    else                       \
    {                          \
        cout << "NO" << endl;  \
    }
#define IFYes(TRUE_OR_FALSE)   \
    if (TRUE_OR_FALSE)         \
    {                          \
        cout << "Yes" << endl; \
    }                          \
    else                       \
    {                          \
        cout << "No" << endl;  \
    }
#define IFyes(TRUE_OR_FALSE)   \
    if (TRUE_OR_FALSE)         \
    {                          \
        cout << "yes" << endl; \
    }                          \
    else                       \
    {                          \
        cout << "no" << endl;  \
    }
#define DEBUG_OUTPUT_ARRAY(XXX, ONE)                            \
    for (int i = 0; i < (ONE); i++)                             \
    {                                                           \
        cout << "DEBUG: i = " << i << " -> " << XXX[i] << endl; \
    }
#define DEBUG_OUTPUT_ARRAY2(XXX, ONE, TWO)                             \
    for (int i = 0; i < (ONE); i++)                                    \
    {                                                                  \
        cout << "<<< i = " << i << " >>>" << endl;                     \
        for (int j = 0; j < (TWO); j++)                                \
        {                                                              \
            cout << "DEBUG: j = " << j << " -> " << XXX[i][j] << endl; \
        }                                                              \
    }
#define DEBUG_OUTPUT_ARRAY2_BOX(XXX, ONE, TWO) \
    for (int i = 0; i < (ONE); i++)            \
    {                                          \
        cout << i << "  ";                     \
        for (int j = 0; j < (TWO); j++)        \
        {                                      \
            cout << XXX[i][j] << " ";          \
        }                                      \
        cout << endl;                          \
    }

typedef pair<long long int, long long int> pll;
typedef pair<long long int, pll> lpll;

const long long int mod = 998244353;
const long long int INF = 1e18;
const long double PI=3.14159265358979323; 
//const long long int pl=1000000;



//union find木(グループわけができる)
class Union_Find{
	public:
	Union_Find(int MAX){
		par =new long long int[MAX+1];
		size =new long long int[MAX+1];
		for (int i = 0; i < MAX; i++) {
			par[i] = i;
            size[i]=1;
		}
	}
	long long int * par;
    long long int * size;
		//木の根を求める
	int unifind_root(int x) {
		if (par[x] == x) {
			return x;
		}
		else {
			return par[x] = unifind_root(par[x]);
		}
	}
		//同じグループか判定
	bool unifind_same(int x, int y) {
		return unifind_root(x) == unifind_root(y);
	}
		//グループ併合
	void unifind_unite(int x, int y) {
		x = unifind_root(x);
		y = unifind_root(y);
		if (x == y) {
			return ;
		}
		par[x] = y;
        size[y] +=size[x];
        return;
	}
};

long long int A[100020],B[100200],C,D,N,M,res=0;
bool used[100200]={};

int main(){
    cout << fixed << setprecision(18);
    
    cin>>N>>M;
    Union_Find UF(N);

    for(long long int i = 0; i < M; i++){
        cin>>A[i]>>B[i];
        A[i]--;
        B[i]--;
        UF.unifind_unite(A[i],B[i]);
    }
    for(long long int i = 0; i < N; i++){
        if(!used[UF.unifind_root(i)]){
            res++;
            used[UF.unifind_root(i)]=true;

        }
    }
    cout<<res-1<<endl;

}



Measured memory_bytes: 6824