#include <stdio.h>
#include <string.h>
 
#define SIZE 1000  // 入力可能な最大文字数
 
// (a) アルファベットかどうかを判定
int isalphabet(char c) {
    return (c >= 'A' && c <= 'Z') || (c >= 'a' && c <= 'z');
}
 
// (b) アルファベットを n 文字ずらす
char caesar(char c, int n) {
    if (c >= 'A' && c <= 'Z') {
        return 'A' + ( (c - 'A' + n + 26) % 26 );
    } else if (c >= 'a' && c <= 'z') {
        return 'a' + ( (c - 'a' + n + 26) % 26 );
    } else {
        return c; // アルファベットでない文字はそのまま
    }
}
 
int main() {
    char input[SIZE];
    char encrypted[SIZE];
    char decrypted[SIZE];
    int n;
 
    // 文字列とシフト量の入力
    printf("暗号化したい文字列を入力してください: ");
    fgets(input, SIZE, stdin);
    input[strcspn(input, "\n")] = '\0';  // 改行文字を除去
 
    printf("シフト量（正の整数）を入力してください: ");
    scanf("%d", &n);
 
    // 暗号化処理
    for (int i = 0; input[i] != '\0'; i++) {
        encrypted[i] = caesar(input[i], n);
    }
    encrypted[strlen(input)] = '\0';
 
    // 復号化処理（-n で元に戻す）
    for (int i = 0; encrypted[i] != '\0'; i++) {
        decrypted[i] = caesar(encrypted[i], -n);
    }
    decrypted[strlen(encrypted)] = '\0';
 
    // 結果表示
    printf("暗号化された文字列: %s\n", encrypted);
    printf("復号化された文字列: %s\n", decrypted);
 
    return 0;
}

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