+---+            +---+           +---+
first | ? |     second | ? |       sum | ? |
      +---+            +---+           +---+



      +----+            +---+          +---+
first | 47 |     second | ? |      sum | ? |
      +----+            +---+          +---+



      +----+            +----+         +---+
first | 47 |     second | 22 |     sum | ? |
      +----+            +----+         +---+



      +----+            +----+         +----+
first | 47 |     second | 21 |     sum | 68 |
      +----+            +----+         +----+