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 does not contain the word 'salesforce', return false

3. 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.