The first solid-state electronic calculator was created in the early 1960s. Pocket-sized devices became available in the 1970s, especially after the Intel 4004, the first microprocessor, was developed by Intel for the Japanese calculator company Busicom. They later became used commonly within the petroleum industry (oil and gas). Find more on wikipedia page...

Let's try to build an elementary calculator.

As an input, you get a sequence of keys pressed, and, as the result of that function, you should show what will be shown on the screen when the last key is pressed.

Build calculator keyboard.

The calculator has a very simple keyboard. All digits (0-9), plus(-), minus(+) and equation sign (=).

Max digit space is 5. It means that the maximum number you can enter is "99999". The calculator ignores the 6th digit you enter. If the result is more than five digits long - "error" is shown as a result.

Input: str

Output: str

assert calculator("12") == "12"  # you see what you enter
assert (
    calculator("12+") == "12"
)  # you see what you enter, one (+) doesn't change anything
assert (
    calculator("12+15") == "15"
)  # last entered number, since the operation is not finished
assert calculator("12+15=") == "27"  # 12 + 15 = 27