Design an algorithm and write its C++ code. that takes one positive integer argument, call it "my_goal" and returns as its value the smallest positive integer n for which 1+2+3+. . . +n is at least equal to my_goal.

The solution must also check that the input is a positive integer and otherwise it should print an error message:

The '9' should outcome '4' because 1+2+3+49 and 1+2+3<9;

The '21' should outcome '6' because 1+2+3+4+5+621 and 1+2+3+4+5<21;

Keep your code commented and indented for ease of reading. Paste your commented codes in the solution and also add a clear screen shot of outcome in your solution.