一个错误必须定义入口点这是什么问题，求大神帮忙

#include<stdio.h>
int mian(void)
{
float a,b;
char c;
printf("please input expression: a(+,-,*,/)b\n");
scanf("%f %f %c",&a,&b,&c);
switch (c)
{
case'+': printf("%f\n",a+b);break;
case'-':printf("%f\n",a-b);break;
case'*':printf("%f\n",a*b);break;
case'/':printf("%f\n",a/b);break;
default:printf("This is error\n");
}
return 0;
}