The first desktop programmable calculators were produced in the mid-1960s. They included the Mathatronics Mathatron (1964) and the Olivetti Programma 101 (late 1965) which were solid-state, desktop, printing, floating point, algebraic entry, programmable, stored-program electronic calculators. Both could be programmed by the end user and print out their results. The Programma 101 saw much wider distribution and had the added feature of offline storage of programs via magnetic cards. Find more on wikipedia page...

In this series of missions you are going 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. Be attentive, it's not always the result of the expression. Actually, playing with the physical calculator or app will really helps you to catch edge cases.

In the sixth mission are added such buttons as capital "B" (backspace) - delete previous character and capital "C" (clean) - delete all output, including error.

Expected behavior:

• beginning zeros should be removed, only-zeros number - converted to single zero;
• among +- signs between numbers, the last one should be taken;
• "==" means repeating the last operation;
• "+=" or "-=" - adding/subtracting the number (or operations result) before the combination (doubling the number/subtracting itself);
• the calculator ignores digit you enter after 5th;
• "-" for numbers < 0 is NOT taking digit place;
• if the abs(result) is more than 99999 - "error" is shown as a result;
• for float, if the integer part of abs(result) is more then 9999 - "error" is shown as a result;
• for float, in case the total length of number is more than 5 digits, it should be rounded to 5 digits (1.23456 -> 1.235);
• for float, beginning and trailing zeros should be removed (until the "." if possible): 0.1200 -> .12 , 123.00 -> 123. . It should be done after the rounding: 1.000123 -> 1. . Stripping of trailing zeros should only be done after entering a number has concluded (non-digit character pressed);
• when after B's no signs remain and new digits are pressed - concatenate with the previous number: 12+B3 -> 123 .

Input: String.

Output: String.

Examples:

assert calculator("123BB") == "1"
assert calculator("123C12*=B") == "14"
assert calculator("123BBBBBB") == "0"
assert calculator("C") == "0"

How it’s used: Calculators are widely used. Understanding the principles of its input and output are interesting and useful.

Precondition: Allowed characters: digits (0-9), dot (.), signs +, -, =, *, /, //, %, ** and their combinations (+=, +-, ==, %= etc.), letters "B" and "C". The integer number may contain no more than 5 digits, float - 4.