import java.util.*;

public class Test {
    public static void main(String args[])
    {
        new Test(new Scanner(System.in));
    }

    Graph g;
    int[] deg;
    boolean[] used;
    int count = 1;
    int ans;

    HashSet<Integer> comp;


    void dfs(int i)
    {
        used[i] = true;
        comp.add(i);

        for(Edge e : g.adj[i])
        {
            if(!used[e.j])
                dfs(e.j);
        }
    }

    Test(Scanner in)
    {
        g = new Graph(in.nextInt()+1);
        used = new boolean[g.N];
        deg = new int[g.N];
        ans = 0;

        for(int i=0; i<g.N; i++)
            comp = new HashSet<>();

        int M = in.nextInt();

        while(M-->0)
        {
            int i = in.nextInt();
            int j = in.nextInt();
            g.add(i, j);
        }

        for(int i=1; i<g.N; i++)
            if (!used[i]) {
                comp.clear();
                dfs(i);
                boolean ok = true;

                for(Integer v : comp) ok &= deg[v] == 2;
                if (ok) ++ans;
            }

        System.out.printf("%d", ans);
    }


    class Graph
    {
        int N, M;

        ArrayList<Edge>[] adj;


        Graph(int NN)
        {
            M = 0;
            adj = new ArrayList[N=NN];
            for(int i=0; i<N; i++)
                adj[i] = new ArrayList<>();
        }

        void add(int i, int j)
        {
            adj[i].add(new Edge(j, M));
            adj[j].add(new Edge(i, M));
            ++deg[i];
            ++deg[j];
            M++;
        }
    }

    class Edge
    {
        int j, id;

        Edge(int jj, int ii) {
            j=jj; id=ii;
        }
    }
}