Upper and lower case letter conversion

The code is as follows ：

#include <stdio.h>
int main(int argc,const char *argv[])
{
    
	char a,b;
	printf(" Please enter ：");
	scanf("%c",&a);
	if(a>='A'&&a<='Z')
		b=a+32;
	else if(a>='a'&&a<='z')
		b=a-32;
	else
	{
    
		printf(" The input is not a letter \n");
		return -1;
	}
	printf(" Convert to :%c\n",b);
	return 0;
}

Running results ：

[email protected]:~/test/c-day3$ ./a.out
 Please enter ：A
 Convert to :a
[email protected]:~/test/c-day3$ ./a.out
 Please enter ：g
 Convert to :G
[email protected]:~/test/c-day3$ ./a.out
 Please enter ：4
 The input is not a letter