#include <stdio.h>
#include <ctype.h>
int main(void)
{
char ch;
printf("Enter some text (type a period to quit).\n");
do {
ch = getchar();
if (islower(ch)) ch = toupper(ch);
else ch = tolower(ch);
putchar(ch);
} while (ch != '.');
return 0;
}
