DynamicJsonを導入しjsonファイルのパースを行っていますが

        dynamic obj = DynamicJson.Parse(json);
        string movie_width= obj.streams.width;

文字入力が違うとエラーが出ます。
どう記述するのが正解なのでしょうか？

json内容

{
    "streams": [
        {
            "index": 0,
            "codec_name": "h264",
            "codec_long_name": "H.264 / AVC / MPEG-4 AVC / MPEG-4 part 10",
            "profile": "Constrained Baseline",
            "codec_type": "video",
            "codec_time_base": "1/48",
            "codec_tag_string": "avc1",
            "codec_tag": "0x31637661",
            "width": 640,
            "height": 360,
            "coded_width": 640,
            "coded_height": 360,
            "has_b_frames": 0,
            "sample_aspect_ratio": "1:1",
            "display_aspect_ratio": "16:9",
            "pix_fmt": "yuv420p",
            "level": 30,
            "chroma_location": "left",
            "refs": 1,
            "is_avc": "true",
            "nal_length_size": "4",
            "r_frame_rate": "24/1",
            "avg_frame_rate": "24/1",
            "time_base": "1/12288",
            "start_pts": 0,
            "start_time": "0.000000",
            "duration_ts": 36864,
            "duration": "3.000000",
            "bit_rate": "351098",
            "bits_per_raw_sample": "8",
            "nb_frames": "72",
            "disposition": {
                "default": 1,
                "dub": 0,
                "original": 0,
                "comment": 0,
                "lyrics": 0,
                "karaoke": 0,
                "forced": 0,
                "hearing_impaired": 0,
                "visual_impaired": 0,
                "clean_effects": 0,
                "attached_pic": 0,
                "timed_thumbnails": 0
            },
            "tags": {
                "language": "und",
                "handler_name": "VideoHandler"
            }
        }
    ]
}