#include<bits/stdc++.h>
using namespace std;

#define endl "\n"
#define LL long long
#define ULL unsigned long long
const LL        INF = 1LL<<59;
const double    PI = 2.0 * acos(0.0);

typedef pair<int,int> pii;
typedef pair<LL,LL> pll;
typedef vector<int> vi;
typedef vector<LL> vl;
typedef vector<pii> vii;
typedef vector<pll> vll;


//// Including Policy Based DS
//#include <ext/pb_ds/assoc_container.hpp>
//#include <ext/pb_ds/tree_policy.hpp>
//using namespace __gnu_pbds;
//
/////cout<<*X.find_by_order(1)<<endl;
/////cout<<X.order_of_key(-5)<<endl;
//
//typedef tree<
//int,
//null_type,
//less< int >,
//rb_tree_tag,
//tree_order_statistics_node_update>
//ordered_set;

#define READ        freopen("in.txt","r",stdin)
#define WRITE       freopen("out.txt","w",stdout)
#define BOOST		ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0)
#define sf 			scanf
#define pf 			printf
#define F  			first
#define S  			second
#define pb 			push_back
#define NL			pf("\n")
#define ALL(v)		v.begin(),v.end()
#define si(x)		sf("%d",&x);
#define sii(x,y)	sf("%d%d",&x,&y);
#define siii(x,y,z)	sf("%d%d%d",&x,&y,&z);
#define sl(x)		sf("%lld",&x);
#define sll(x,y)	sf("%lld%lld",&x,&y);
#define slll(x,y,z)	sf("%lld%lld%lld",&x,&y,&z);
#define sqr(x)      ( (x) * (x) )
#define mem(x,y) 	memset(x,y,sizeof x )
#define ch 			printf("Came Here!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!\n")
#define deb(x)      cerr<<#x<<" :: "<<x<<"\n"
#define SV(v)		sort(v.begin(),v.end())
#define SVR(v,cmp)	sort(v.begin(),v.end(),cmp)
#define FOR(i,n)	for(int i=0;i<n;i++)
#define cnd			tree[idx]
#define lc			tree[idx*2]
#define rc			tree[idx*2+1]
#define lnd 		(2*idx),(b),( (b+e) /2 )
#define rnd 		((2*idx)+1),(((b+e)/2)+1),(e)

// graph direction array [4]
int X[4]={0,0,-1,1};
int Y[4]={1,-1,0,0};
// graph direction array [8]
//int X[8]={0,0,1,-1,-1,1,1,-1};
//int Y[8]={1,-1,0,0,-1,-1,1,1};
// Bishop direction array [8]
//int BX[8]={0,0,1,-1,-1,1,1,-1};
//int BY[8]={1,-1,0,0,-1,-1,1,1};
// Knight Direction Array
//int KX[8] = {1,1,2,2,-1,-1,-2,-2};
//int KY[8] = {2,-2,1,-1,2,-2,1,-1};

// Bit Operations
#define popcountL   __builtin_popcountll
#define popcount    __builtin_popcount
inline bool checkBit(int N,int pos){return (bool)(N & (1<<pos));} // check bit
inline int setBit(int N,int pos){ return N=N | (1<<pos);}// set bit
inline int unsetBit(int N,int pos){	return N=( N & ( ~( 1<<pos ) ) );}// unset bit
inline int toggleBit(int N,int pos){ return N = ( N ^(1<<pos) ); }// toggle bit


inline LL modMul(LL a, LL b,LL mod){
    LL ans = 0;
    a = a % mod;
    while (b > 0){
        if ( b % 2 )ans = (ans%mod+ a%mod) % mod;
        a = (a%mod * 2%mod) % mod;
        b /= 2;
    }
    return ans % mod;
}
inline LL powerMod(LL a,LL b,LL mod){
	if(b==0)return 1LL%mod;
	LL x=powerMod( a,b/2,mod );
	x = ( x%mod * x%mod ) % mod;
	if( b%2==1 )x = ( x%mod * a%mod ) % mod;
	return x%mod;
}
inline LL lcm( LL a,LL b ){
	LL g = __gcd(a,b);
	return ((a/g) * b);
}

//-------------------------------------------------------------------------------
//-------------------------------------------------------------------------------
const int mx = 1e5+5;

bool vis[mx];
vi g[mx];

void dfs(int u){
    vis[u] = 1;
    for(int v:g[u]){
        if( !vis[v] )dfs(v);
    }
}


int main(){
    BOOST;
    #ifdef MujahidPC
        double start = clock();
//        READ;
//        WRITE;
    #endif
    int t,n,m;
    cin>>n>>m;
    FOR(i,m){
        int u,v;cin>>u>>v;
        g[u].pb(v);
        g[v].pb(u);
    }

    int cnt = 0;
    for(int i=1;i<=n;i++){
        if( !vis[i] ){
            cnt++;
            dfs(i);
        }
    }

    cout<<(cnt-1)<<endl;


    #ifdef MujahidPC
        double tim = (clock() - start)/CLOCKS_PER_SEC;
        cerr<<"Running Time : "<<tim<<" \n";
    #endif
    return 0;
}

Measured memory_bytes: 9712