#include <bits/stdc++.h>

using namespace std;
int n,m,x,y;
vector<int>nod[100005];
bitset<100005>ap;
deque<int>dq;
int main()
{
    freopen("dfs.in","r",stdin);
    freopen("dfs.out","w",stdout);
    scanf("%d%d",&n,&m);
    for(int i=1;i<=m;i++)
    {
        scanf("%d%d",&x,&y);
        nod[x].push_back(y);
        nod[y].push_back(x);
    }
    int pos=1;
    int howmany=0;
    int ans=0;
    while(howmany<n)
    {
        ans++;
        while(ap[pos])
            pos++;
        dq.push_back(pos);
        ap[pos]=1;
        howmany++;
        while(!dq.empty())
        {
            int ind=dq.front();
            dq.pop_front();
            for(int i=0;i<nod[ind].size();i++)
            {
                if(ap[nod[ind][i]])
                    continue;
                howmany++;
                ap[nod[ind][i]]=1;
                dq.push_back(nod[ind][i]);
            }
        }
    }
    printf("%d",ans);
    return 0;
}