def edge(a,b,n,L=[]):
    for k in range(1,n+1):
        if a !=0 or b != 0:
            if a-k >= 0:
                entry = {(min(a,b) , max(a,b)) : (min(a-k,b) , max(a-k,b))}
                if entry not in L:
                    L.append(entry)
                    L = edge(a-k,b,n,L)
            if b-k >=0:
                entry = {(min(a,b) , max(a,b)) : (min(a,b-k) , max(a,b-k))}
                if entry not in L:
                    L.append(entry)
                    L = edge(a,b-k,n,L)
    return L