#include <stdio.h>
#include <stdbool.h>

int rot13(int);
bool is_lowcase(int);
bool is_upcase(int);

int main(int argc, char *argv[])
{
    int b, r;
    printf("Enter a number to see its rot13: ");
    scanf("%c", &b);
    r = (char)rot13(b);
    printf("The rot13 of %c is %c.\n", b, r);

    return 0;
}

int rot13(int c)
{
    int k;
    if (is_upcase(c)) {
        if (c > 'M') {
            c -= 13;
        }
        else {
            c += 13;
        }
    }
    else if (is_lowcase(c)) {
        if (c > 'm') {
            c -= 13;
        }
        else {
            c += 13;
        }
    }
    else {
        return c;
    }
}

bool is_lowcase(int c)
{
    if (c >= 'a' && c <= 'z') {
        return true;
    }
    else {
        return false;
    }
}

bool is_upcase(int c)
{
    if (c >= 'A' && c <= 'Z') {
        return true;
    }
    else {
        return false;
    }
}