Addition +1

Front 1

0+1

Back 1

1

Front 2

1+1

Back 2

2

Front 3

2+1

Back 3

3

Front 4

3+1

Back 4

4

Front 5

4+1

Back 5

5

Front 6

5+1

Back 6

6

Front 7

6+1

Back 7

7

Front 8

7+1

Back 8

8

Front 9

8+1

Back 9

9

Front 10

9+1

Back 10

10

Front 11

1+9

Back 11

10

Front 12

1+8

Back 12

9

Front 13

1+7

Back 13

8

Front 14

1+6

Back 14

7

Front 15

1+5

Back 15

6

Front 16

1+4

Back 16

5

Front 17

1+3

Back 17

4

Front 18

1+2

Back 18

3