总是报错：[Error] invalid conversion from ‘const char*’ to ‘char’ [-fpermissive]，我该怎样改呢？

#include <stdio.h> 

int main(){
	char a,b;
	a="a";
	b="b";
	a=a-32;
	b=b-32;
	printf("%c,%c\n %d,%d\n",a,b,a,b); 
}