At IBM, candidates are usually required to undergo four steps in the recruitment process. These assessments evaluate a candidate's technical knowledge as well as their analytical abilities. The steps are as follows:

• Cognitive Ability Assessment
• Coding Round
• Technical Interview
• HR Interview

Each of these rounds is eliminative. To receive a job offer from IBM, you must clear all of the above rounds.

Write a program to convert the characters of a string into the opposite case. That is, if a character is lowercase, convert it to uppercase, and vice versa.