t=raw_input().split(" ")
nr=1
for i in range(0,3):
    t[i]=int(t[i])
if (t[1]==t[2] and t[0]%2==0):
    print -1
else:
    v=t[0]-t[2]
    print v
    i=1
    contor=1
    contor2=1
    while v!=0:
        contor+=1
        contor2+=1
        print str(i)+" "+str(i+1)
        v=v-1
        if(i<t[0]-1):
            if(contor==(t[0]/t[1]) and i<t[0]-2):
                if(t[0]-contor2>=2*t[0]/t[1]):
                        i+=2
                        contor=0
                        contor2+=1
            else:
                i+=1
                contor+=1
                contor2+=1