这个程序如果除数为0时，则退出程序应该怎么实现，为什么我输0时，结束不了这个程序

#include<stdio.h>
double Add(int a, int b)
{
return a + b;
}
double Sub(double a, double b)
{
return a - b;
}
double Mul(double a, double b)
{
return a * b;
}
double Div(double a, double b)
{
return a / b;
}

void menu()
{
printf("***** 1.add 2.sub \n");
printf(" 3.mul 4.div \n");
printf("* 0.exit *****\n");
}
int main()
{
int input = 1;
char op;
double x = 0;
double y = 0;
double ret = 0;
while (input)
{
menu();
printf("请选择：");
scanf("%d", &input);
if (input >= 1 && input <= 4)
{
printf("输入一个计算式：");
scanf("%lf%c%lf", &x,&op,&y);
}
switch (input)
{
case 1:
ret = Add(x, y);
break;
case 2:
ret = Sub(x, y);
break;
case 3:
ret = Mul(x, y);
break;
case 4:
else
ret = Div(x, y);
break;
case 0:
printf("退出\n");
break;
default:
printf("输入错误\n");
break;
}
printf("ret = %.1f\n", ret);
}
return 0;
}

        case 4:
            if(y == 0) return;
            ret = Div(x, y);
            break;