Math.random() question regarding {low input} and {high input}: What's the difference between no ' +1 ' and +1?

// Collect inputs from a user
let input = prompt('Please provide a low number:');
let input2 = prompt('Please provide a high number:');

// Convert each input to a number
input = parseInt(input);
input2 = parseInt(input2);

if (input && input2) {
  // Use Math.random() and the user's number to generate a random number

   // **MY ORIGINAL solution** -- no plus 1
   //const randNum = Math.floor(Math.random() * (input2 - input1)) + input;

   //  **VIDEO SOLUTION guidelines**
   const randNum = Math.floor(Math.random() * (input2 - input1 + 1)) + input;

  // Log a message displaying the random number generated between first and second num input
  console.log(`${randNum} is a random number between ${input} and ${input2}.`);
  // otherwise
} else {
  // Log error message
  console.log('Invalid number(s) input. Please try again.');
}