Program won't enter for loop ( C language) : Hello I am pretty new to programming.

I know that there is a lot of already asked questions on this topic, but i read them all and  still can't figure where is the problem. The assignment is to write programm that will take 4-digit number and multiply those for digits( exp. 1576; 1*5*6*7). The programm runs bat ok untill for loop and then just stop(doesn't even say that there is some error). Thanks in advance for your help

    #include< stdio.h >
    #include <math.h>
    main()
    {
	 int n,i,b=4,proizvod=1 ; 
	 printf("Unesite prirodan cetvorocifreni broj:");
	 scanf_s("%d ", &n);
	 for (i = 0; i < b; b--) {
		 proizvod = proizvod * n % 10;
		 n = n / 10;
	 }
	  printf("Prozivod cifara je: %d", proizvod);
    }