#include <fstream>
#include <vector>
#include <queue>

using namespace std;

ifstream in("dfs.in");
ofstream out("dfs.out");

vector<int> a[100005];
int n,m;
int marcaj[100005];

void DFS(int nod)
{
    marcaj[nod]=1;
    for(int i=0;i<a[nod].size();i++)
    {
        if(!(marcaj[a[nod][i]]))
            DFS(a[nod][i]);
    }
}

int main()
{
    int i,x,y;
    int nr=0;
    in>>n>>m;
    for(i=1;i<=m;i++){
        in>>x>>y;
        a[x].push_back(y);
    }
    for(i=0;i<=n;i++)
    {
        if(!(marcaj[i]))
        {
            DFS(i);
            nr++;
        }
    }
    out<<nr-1;
    in.close();
    out.close();
    return 0;
}