Math calculator

#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#include <string.h>
#include <math.h>

/*
Math calculator by Goran
*/

int main()
{
    char answer[15]; // char answer can store 15 bytes for answer.
    float firstNumber, secondNumber; // floats for first and second number.

    printf("What operation do you want to use ?\n- Addition\n- Subtraction\n- Multiplication\n- Division\n\n"); // asking user what operation do you want to use. User can answer by writing only first letter or full name of operation.
    scanf(" %s", answer); // gets input from user and stores it into char answer.
    answer[0] = toupper(answer[0]); // making first letter user input uppercase if it's already not.

    if(!(answer[0] == 'A' || answer[0] == 'S' || answer[0] == 'M' || answer[0] == 'D')){ // if answer is not A, S, M or D code below gets executed.
        printf("\n"); // prints blank space.
    }

    while(!(answer[0] == 'A' || answer[0] == 'S' || answer[0] == 'M' || answer[0] == 'D')){ // if answer is not A, S, M or D code below gets executed until answer is A, S, M or D.
        printf("You need to enter correct operation: "); // prints message to user to input correct operation name.
        scanf(" %s", answer); // gets input from user and stores it into char answer.
        answer[0] = toupper(answer[0]); // making first letter user input uppercase if it's already not.
    }

    switch(answer[0]){ // testing if first character.
        case 'A': // if first character is letter A code below gets executed.
            printf("\nEnter first number: "); // prints message to user to input first number.
            scanf(" %f", &firstNumber); // gets input from user and stores it into float firstNumber.
            printf("Enter second number: "); // prints message to user to input second number.
            scanf(" %f", &secondNumber); // gets input from user and stores it into float secondNumber.
            printf("Result: %.2f + %.2f = %.2f \n", firstNumber, secondNumber, firstNumber + secondNumber); // prints result of equation.
            break;
        case 'S': // if first character is letter S code below gets executed.
            printf("\nEnter first number: "); // prints message to user to input first number.
            scanf(" %f", &firstNumber); // gets input from user and stores it into float firstNumber.
            printf("Enter second number: "); // prints message to user to input second number.
            scanf(" %f", &secondNumber); // gets input from user and stores it into float secondNumber.
            printf("Result: %.2f - %.2f = %.2f \n", firstNumber, secondNumber, firstNumber - secondNumber); // prints result of equation.
            break;
        case 'M':  // if first character is letter M code below gets executed.
            printf("\nEnter first number: "); // prints message to user to input first number
            scanf(" %f", &firstNumber); // gets input from user and stores it into float firstNumber.
            printf("Enter second number: "); // prints message to user to input second number.
            scanf(" %f", &secondNumber); // gets input from user and stores it into float secondNumber.
            printf("Result: %.2f * %.2f = %.2f \n", firstNumber, secondNumber, firstNumber * secondNumber); // prints result of equation.
            break;
        case 'D':  // if first character is letter D code below gets executed.
            printf("\nEnter first number: "); // prints message to user to input first number
            scanf(" %f", &firstNumber); // gets input from user and stores it into float firstNumber.
            printf("Enter second number: "); // prints message to user to input second number.
            scanf(" %f", &secondNumber); // gets input from user and stores it into float secondNumber.
            printf("Result: %.2f / %.2f = %.2f \n", firstNumber, secondNumber, firstNumber / secondNumber); // prints result of equation.
    }

    system("pause"); // pausing program after executing.
    return 0;
}