Complete the given apex method which accepts a string input to verify if the given input contains the word 'salesforce' (case sensitive).

Instructions:

1. If the input contains the word 'salesforce', return true

2. If the input contains the word 'Salesforce', return false (case sensitive).

3. If the input does not contain the word 'salesforce', return false

4. If the input is blank or null or a white space, return false

Example:

String input = 'salesforce service cloud';
Boolean result = stringCheckController.keywordExist(input); //Result is true

Note: String is a primitive data type in apex. Apex provides a set of built in functions to operate on strings. See the official documentation for apex strings and utilize them to solve this scenario.