#include <bits/stdc++.h>

using namespace std;

vector <int> a[100005];
bool v[100005];
int n,m,x,y,t;

void viz(int nod, bool init){
    v[nod]=1;
    for(int i=1;i<=a[nod].size();i++){
        if(v[a[nod][i]]==0) viz(a[nod][i],false);
    }
    if(init){
        t++;
        int i=nod+1;
        while(v[i]==1 && i<=n) i++;
        if(i<=n) viz(i,true);
    }
}

int main()
{
    ifstream in("dfs.in");
    ofstream out("dfs.out");
    in >> n >> m;
    for(int i=1;i<=m;i++){
        in >> x >> y;
        a[x].push_back(y);
        a[y].push_back(x);
    }
    viz(1,true);
    out << t;
}