caesar cipher

Help ! saat key saya masukan angka 3, outputnya sesuai tapi pas ganti dengan 18, seperti digambar

salahnya dimana ya ?

printf("Enter a message to encrypt: ");
    gets(message);

    printf("Enter key :");
    scanf("%d", &key);
    for(i = 0; message[i] != '\0'; ++i){
        ch = message[i];

        if(ch >= 'a' && ch <= 'z'){
            ch = ch + key;

            if(ch > 'z'){
                ch = ch - 'z' + 'a'-1;
            }

            message[i] = ch;
        }
        else if(ch >= 'A' && ch <= 'Z'){
            ch = ch + key;

            if(ch > 'Z'){
                ch = ch - 'Z' + 'A'-1;
            }

            message[i] = ch;
        }
    }

Sebelumnya Terima kasih ^_^