So I am trying to make my first basic calculator

my problem is in "case 2" which was suppose to subtract two numbers is adding them even though i used "-" in between them

here is the program

import java.util.Scanner; public class MyClass { public static void main(String args[]) { Scanner input = new Scanner(System.in); int s1=0, calculation = 0; double sum = 0, sum1 = 1; int o; String s; do { do { System.out.println("enter the quantity of numbers to calculate"); s1 = input.nextInt(); if (s1<=1) { System.out.println("please enter two number minimum"); } }while (s1<=1); /* this means that if we have only 1 number to calculate we cant do that so we ask user again to enter the number */ int quantity[]= new int[s1]; System.out.println("enter the numbers"); for (int i=0;i<s1;i++) /*this means quantity[0] is the first empty box to input the first number, and its gonna continue till it reaches s1 number of quantity.*/ { quantity[i] = input.nextInt(); } if (s1<=2) /*we are making seperate condition for division and subtraction cuz they only use two numbers*/ { System.out.println("choose the method of calculation"); System.out.println("type 1 for division"); System.out.println("type 2 for subtraction"); calculation = input.nextInt(); } else { System.out.println("choose the method of calculation"); System.out.println("type 3 for multiplication"); System.out.println("type 4 for addition"); calculation = input.nextInt(); } switch (calculation) { case 1: for (int i=0;i<2;i++) { sum = quantity[i] / quantity[i++]; /* this means that its gonna its gonna take the number stored in quantity[0](in first run) and divide it with the value stored in quantity[1] */ } case 2: for (int i=0;i<2;i++) { sum = quantity[i] - quantity[i++]; } case 3: for (int i=0;i<s1;i++) { sum1 = quantity[i] * sum1; /* we are taking sum1=1 from main class because if we use sum whose value is 0 the result is alwasy gonna be 0 since anythngX0=0*/ } case 4: for (int i=0;i<s1;i++ ) { sum = quantity[i] + sum; } } if ( (calculation)==3) /* since multiplication use variable sum1 to store the sum we are gonna have to make a seperate condition to print it*/ { System.out.println("the sum of "); for (int i=0;i<s1;i++) { System.out.print(quantity[i]+", "); /* this code will print "the sum of" first and then loop itself by printing the numbers stored in quantity[0] (in first run) and then print quantity[1] in the same line till we reach the last quanity i.e s1 */ } System.out.println("is"+sum1); } else { System.out.println("the result is"+ sum); } System.out.println("do you want to calculate again?"); s = input.next(); } while (s.equalsIgnoreCase("yes") || (s.equalsIgnoreCase("yep") || (s.equalsIgnoreCase("yea") || (s.equalsIgnoreCase("ok") || (s.equalsIgnoreCase("okay")))))); } }