//----AUTHOR:kkdrummer----/
#include<bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
//#include <boost/multiprecision/cpp_int.hpp> 
//using namespace boost::multiprecision; 
using namespace __gnu_pbds;
using namespace std;
typedef    long long                	 ll;
typedef    long double              	 ld;
typedef    unordered_set<ll>         	 usll;
typedef    unordered_multiset<ll>    	 umsll;
typedef    multiset<ll>              	 msll;
typedef    set<ll>                  	 sll;
typedef    vector<ll>               	 vll;
typedef    pair<ll,ll>              	 pll;
typedef    vector<pll>              	 vpll;
typedef    priority_queue<ll>       	 pqll;
typedef    vector<int>              	 vi;
typedef    set<int>                 	 si;
typedef    multiset<int>            	 msi;
typedef    unordered_multiset<int>  	 umsi;
typedef    unordered_set<int>       	 usi;
typedef    pair<int,int>               	 pi;
typedef    vector<pi>                	 vpi;
typedef    priority_queue<int>       	 pqi;
typedef    tree<int,null_type,less<int>,rb_tree_tag,tree_order_statistics_node_update>   ind_si;
typedef    tree<ll,null_type,less<ll>,rb_tree_tag,tree_order_statistics_node_update>   ind_sll;
typedef    tree<int,null_type,less_equal<int>,rb_tree_tag,tree_order_statistics_node_update>   ind_msi;
typedef    tree<ll,null_type,less_equal<ll>,rb_tree_tag,tree_order_statistics_node_update>   ind_msll;
#define    in          		insert
#define    fi          		first
#define    se          		second
#define    pb   	    	push_back
#define    mp   	   		make_pair
#define    be   	   		begin
#define    en   	   		end
#define    itr  	   		iterator
#define    io  	       		ios_base::sync_with_stdio(false); cin.tie(NULL); cout.tie(NULL);
#define    mo   	   		998244353
#define    inf  	   		8222372026854775807
#define    ninf   	   		-inf
#define    ima   	   		1147483647
#define    imi   	   		-ima
#define    oncnt       		__builtin_popcount
#define    zerobegin   		__builtin_clz
#define    zeroend    		__builtin_ctz
#define    parity      		__builtin_parity
#define    all(x)      		x.be(),x.en()
#define    eps         		1e-9
#define    coutd       		cout<<setprecision(10)<<fixed
#define    mems(dp,x)  		memset(dp,x,sizeof(dp))
#define    fbo         		find_by_order
#define    ook    	   		order_of_key
#define    upb    	   		upper_bound
#define    lowb  	   		lower_bound 
#define    lte(v,x)    		(upb(all(v),x)-v.be())
#define    gte(v,x)    		(v.end()-lowb(all(v),x))
#define    gt(v,x)     		(v.en()-upb(all(v),x))
#define    lt(v,x)     		(lowb(all(v),x)-v.be())
const      ld PI= 			3.1415926535897932384626433832792884197169399375105820974944;
inline ll modpow(ll x,ll n){if(n==0)return 1;if(x==0)return 0;if(n==1)return(x%mo);ll u=(modpow(x,n/2));u=(u*u)%mo;if(n%2!=0)u=(u*x%mo)%mo;return u;}
inline ll modinv(ll x){return modpow(x,mo-2);}
inline ll mmul(ll a,ll b){if(a>=mo)a=a%mo;if(b>=mo)b=b%mo;if(a*b>=mo)return(a*b)%mo;return(a*b);}
inline ll madd(ll a, ll b){if(a>=mo)a=a%mo;if(b>=mo)b=b%mo;if(a+b>=mo)return(a+b)%mo;return(a+b);}
inline ll msub(ll a, ll b){if(a>=mo)a=a%mo;if(b>=mo)b=b%mo;return(((a-b)%mo+mo)%mo);}
inline ll mdiv(ll a,ll bb){if(a>=mo)a=a%mo;ll b=modinv(bb);if(b>=mo)b=b%mo;if(a*b>=mo)return(a*b)%mo;return(a*b);}
inline ll gcd(ll a,ll b){return __gcd(a,b);}
inline ll lcm(ll a,ll b){return (a*b)/gcd(a,b);}
 


vi ad[200001]; 
vi adjr[200001];
bool visited[200001]={false};
 int n,m;

void dfsfillorder (stack<int> &s,int u)
{
if(visited[u]) return;
visited[u]=true;
for(int i=0;i<ad[u].size();i++)
dfsfillorder(s,ad[u][i]);
s.push(u);
}

void rev()
{
for(int i=1;i<=n;i++)
for(int j=0;j<ad[i].size();j++)
adjr[ad[i][j]].pb(i);  
}

void dfsutil(int x,vi &vx)
{
	if(visited[x]) return ;
	vx.pb(x);
		visited[x]=true;
		for(int i=0;i<adjr[x].size();i++)
		dfsutil(adjr[x][i],vx);				
}

vector<vector<int> > getSCC(int v)
{
	vector<vector<int> > cc;
	stack<int> s;
	for(int i=1;i<=v;i++)
	dfsfillorder(s,i);

	rev();

	for(int i=1;i<=v;i++)
	visited[i]=false;
	
	while(!s.empty())
	{   int x=s.top();
		s.pop();
		if(!visited[x])
		{
		vi vx;
		dfsutil(x,vx);
		 cc.pb(vx);
	    } 
	}

return cc;		
}





int main()
{               io 
int testcases=1;            //           cin>>testcases;
while(testcases--)
{

cin>>n>>m;
for(int i=0;i<m;i++)
{
	int a,b;
	cin>>a>>b;
	ad[a].pb(b);
	ad[b].pb(a);
}

vector<vector<int> > cc=getSCC(n);
cout<<cc.size()-1;


















































}return 0;}

Measured memory_bytes: 21496