Four arithmetic operations in c language Why is the result of my operation incorrect?

Addition: void main(){ int a,b,z;char ch;printf("Please enter the expression:");scanf("%d%c%d",&a,&ch,&b);switch (ch){ case'+':z=a+b;break; case'-':z=ab;break; case'*':z=a*b;break; case'/':if(b) z=a/b;else printf("Divisor cannot be 0"); break;} printf("%d",z); return 0;}