forairy.c

#include<stdio.h>

int main(void){

    int c;
    int e;

    while((c=getchar()) !=EOF){
        if(c >='A' && c<='Z'){
            if((e = c + 13) <= 'Z'){
                putchar(e);
            }
        else{
            e = c - 13;
            putchar(e);
        }
    } 
    else if(c >='a' && c <='z'){
        if((e=c + 13) <= 'z'){
            putchar(e);
        }
        else{
            e = c - 13;
            putchar(e);
        }
    }
    else{
        putchar(c);
    }
    }
    return 0;
}