c
a
d
b
a
a
b
a
a
d
a
a
c
c
d
d
a
d
b
c
b
c
b
b
d
a
b
d
a
b
c
d
d
b
c
c
d
b
c
a
b
b
c
d
a
c
d
a
a
c