Working with nested lists in R - json

Question

I have dataset in json format. When I pull this data to a variable in R using jsonlite::fromJson, a list is created. This list further contains another list, which in turn contains another list and final list has other objects.
How do I make a dataframe out of this data?
Sample dataset:
{
"AG2": {
"-KB89nmaAHtjvIHl6X6I": {
"attempts": 0,
"isCorrect": true,
"questionIndex": 0,
"score": 20,
"timeBonus": 95,
"timestamp": "Mon Feb 22 2016 18:59:17 GMT+0530 (India Standard Time)",
"totalScore": 20
},
"-KB89q8EJ-rYvjzVcIiQ": {
"attempts": 0,
"isCorrect": true,
"questionIndex": 1,
"score": 39,
"timeBonus": 93,
"timestamp": "Mon Feb 22 2016 18:59:27 GMT+0530 (India Standard Time)",
"totalScore": 59
},
"-KB89s7SVjq0YZQ5XrOt": {
"attempts": 0,
"isCorrect": true,
"questionIndex": 2,
"score": 59,
"timeBonus": 95,
"timestamp": "Mon Feb 22 2016 18:59:35 GMT+0530 (India Standard Time)",
"totalScore": 118
},
"-KB89xsifcqbxCDE9Ygt": {
"attempts": 0,
"isCorrect": true,
"questionIndex": 3,
"score": 78,
"timeBonus": 93,
"timestamp": "Mon Feb 22 2016 18:59:59 GMT+0530 (India Standard Time)",
"totalScore": 196
},
"-KB8A-85L2P9nGmii3L_": {
"attempts": 1,
"isCorrect": "The output value should be your name",
"questionIndex": 4,
"score": 0,
"timeBonus": 95,
"timestamp": "Mon Feb 22 2016 19:00:08 GMT+0530 (India Standard Time)",
"totalScore": 196
},
"-KB8A1JTmwee2a5gRkLq": {
"attempts": 1,
"isCorrect": true,
"questionIndex": 4,
"score": 85,
"timeBonus": 87,
"timestamp": "Mon Feb 22 2016 19:00:17 GMT+0530 (India Standard Time)",
"totalScore": 281
}
},
"AKD": {
"-KB88bypMqEu3Y2zOEKD": {
"attempts": 0,
"isCorrect": true,
"questionIndex": 0,
"score": 19,
"timeBonus": 90,
"timestamp": "Mon Feb 22 2016 18:54:07 GMT+0530 (India Standard Time)",
"totalScore": 19
},
"-KB88grpZUs3j3J4SNWc": {
"attempts": 0,
"isCorrect": true,
"questionIndex": 1,
"score": 37,
"timeBonus": 85,
"timestamp": "Mon Feb 22 2016 18:54:27 GMT+0530 (India Standard Time)",
"totalScore": 56
},
"-KB88jYngOMf7chyXUch": {
"attempts": 0,
"isCorrect": true,
"questionIndex": 2,
"score": 58,
"timeBonus": 91,
"timestamp": "Mon Feb 22 2016 18:54:38 GMT+0530 (India Standard Time)",
"totalScore": 114
},
"-KB88mtpsbwp6GD4VQ0f": {
"attempts": 0,
"isCorrect": true,
"questionIndex": 3,
"score": 76,
"timeBonus": 89,
"timestamp": "Mon Feb 22 2016 18:54:52 GMT+0530 (India Standard Time)",
"totalScore": 190
},
"-KB88z1nL0Ckhn5A5Lsq": {
"attempts": 1,
"isCorrect": "The output value should be your name",
"questionIndex": 4,
"score": 0,
"timeBonus": 60,
"timestamp": "Mon Feb 22 2016 18:55:41 GMT+0530 (India Standard Time)",
"totalScore": 190
}
}
}
Required dataframe:
User | TimeStamp | Question Index | Attempts | TimeBonus | isCorrect | Score | Total
Thanks.