C :

#include<stdlib.h>
#include<stdio.h>
int main(){
	char a,b;
	scanf("%c",&a);
	if('A'<=a&&a<='Z'){
         b=a+32;
		printf("%c\n",b);
	}
	else
	if('a'<=a&&a<='z')
		printf("%c\n",a-32);
	else
	
		printf("No\n");
	return 0;
}

Pascal :

var
w:integer;
q:char;
 begin
 readln(q);
 w:=ord(q);
 if (w>=65) and (w<=90) then
 write(chr(w+32))
 else if (w>=97) and (w<=120) then
 write(chr(w-32))
 else write('No'); 
 end.