Tenhle program by mel prevadet velka pismena na mala a naopak, nevite proc mi to hazi error "case label does not reduce to an integer constant"?

#include <stdio.h>

int main(){
	char a;
	char b;
	
	scanf("%c", &a);
	switch(a){
		case a<91:
			b = a + 32;
		break;
		case a>97:
			b = a - 32;
		break;
	}
	printf("%c", b);
}