class GraphMatrix {
  // simple graph (no multiple edges); undirected; unweighted
  private int numVertices;
  private int numEdges;
  
  private boolean[][] adjMatrix;
  
  public GraphMatrix(int n) {
    numVertices = n;
    numEdges = 0;
    adjMatrix = new boolean[n][n];
  } // end constructor
  
  public int getNumVertices() {
    return numVertices;
  } // end getNumVertices
  
  public int getNumEdges() {
    return numEdges;
  } // end getNumEdges
  
  public boolean isEdge(int v, int w) {
    return adjMatrix[v][w];
  } // end getWeight

  public void addEdge(int v, int w) {
    if (!isEdge(v,w)) {
      adjMatrix[v][w] = true;
      adjMatrix[w][v] = true;
      numEdges++;
    }
  } // end addEdge
    
  public void removeEdge(int v, int w) {
    if (isEdge(v,w)) {
      adjMatrix[v][w] = false;
      adjMatrix[w][v] = false;
      numEdges--;
    }
  } // end remove
  
  public int nextAdjacent(int v,int w)
  // if w<0, return the first adjacent vertex
  // otherwise, return next one after w
  // if none exist, return -1
  {
    for (int i=w+1; i<numVertices; i++)
      if (isEdge(v,i))
        return i;
    return -1;
  }
} // end Graph