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==1):
    print "Impossible"

v=t[0]-t[2]
print v
i=1
while v!=0:
    print str(i)+" "+str(i+1)
    i+=1
    v-=1