A farmer has $20\text{m}$ of fencing to build a pen. The pen will be built against a wall in a rectangular shape, and therefore will have three sides, with two of those sides being equal.

The farmer needs to leave a $1\text{m}$ gap on the side opposite the wall where a gate will be constructed.

Find, in ${\text{m}}^2$, the largest possible area for the pen.

Suppose $y\text{m}$ of fencing is used for the side opposite the wall, and the other two sides use $x\text{m}$ each.

The area of the rectangle would be $A=x(y+1)$.

Since the total amount of fencing available is $20\text{m}$, we would have $2x+y=20$.

Give your whole answer as a decimal.