Caesar

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>

#define MAX 100

void encode(char *s, int length, int k)
{
    int n;
    for (int i = 0; i < length; ++i)
    {
        if (!isalpha(s[i])) continue;
        s[i] = toupper(s[i]);
        n = s[i];
        n += k;
        while (n > 'Z')
            n -= 26;
        s[i] = (char)n;
    }
}

void decode(char *s, int length, int k)
{
    int n;
    for (int i = 0; i < length; ++i)
    {
        if (!isalpha(s[i])) continue;
        s[i] = toupper(s[i]);
        n = s[i];
        n -= k;
        while (n < 'A')
            n += 26;
        s[i] = (char)n;
    }
}

int main()
{
    FILE *infile = fopen("Cau3.INP", "r");
    FILE *outfile = fopen("Cau3.OUT", "w");

    int n;
    fscanf(infile, "%d", &n);

    // Clear input buffer
    while (fgetc(infile) != '\n');

    char s1[MAX];
    char s2[MAX];

    fgets(s1, MAX, infile);
    fgets(s2, MAX, infile);

    int l1 = strlen(s1);
    int l2 = strlen(s2);

    encode(s1, l1, n);
    decode(s2, l2, n);

    fprintf(outfile, "%s", s1);
    fprintf(outfile, "%s", s2);

    fclose(infile);
    fclose(outfile);
}