Someone

#include <stdio.h>

int main(void) {
	char ch;
	while((ch = getchar()) != -1) {
		/* a-z or A-Z? */
		if((ch > 64 && ch < 91) || (ch > 96 && ch < 123)) {
			/* < N or < n? */
			if(ch < 78 || (ch > 96 && ch < 110)) {
				putchar(ch+13);
			}else{
				putchar(ch-13);
			}
		}else{
			/* anything else */
			putchar(ch);
		}
	}
}