Simple calculator that respects operation priority

I wrote the code for a calculator, but it's not respecting the priority of operation. What do I need to change? The program doese't have to support (). When you run it, you should enter a number, then a sign and continue reading numbers and signs until user input is =.

public static void main(String[] args) {
 Scanner sc = new Scanner(System.in);
int a,c ;
 char sgn;

 c = sc.nextInt();

do{     
    sgn = sc.next().charAt(0); 
    if (sgn=='=') break;

    a = sc.nextInt();
 switch (sgn) {
       case '+' : c += a; break;    
       case '-' : c -= a; break;
       case '*' : c *= a; break;
       case '/' : c /= a; break;
       }
   }while(sgn!='=');
  System.out.println(c);
 }
}

Ok, solved by myself

public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int prod = sc.nextInt();
    int sum = 0;
    char ops;

    do {
        ops = sc.next().charAt(0);
        switch (ops) {
        case '*':
            prod *= sc.nextInt();
            break;
        case '/':
            prod /= sc.nextInt();
            break;
        case '+':
            sum += prod;
            prod = sc.nextInt();
            break;
        case '-':
            sum += prod;
            prod = -sc.nextInt();
            break;
        }
    } while (ops != '=');

    System.out.println(sum + prod);
}