How many of Dan Brown's books does Professor Robert Langdon appear in?
5
1
1
1
1
2
1
2
3
1
2
3
1
2
3
1
2
3
1
2
3
1
2
4
3
1
2
4
3
1
2
4
3
1
2
4
3
1
2
5
4
3
1
2
5
4
3
1
2
5
4
3
1
2
5
4
3
1
2
5
4
3
1
2
5
4
3
1
2
5
4
3
2
5
4
0
3
2
5
4
3
0
2
5
4
3
0
5
4
3
5
4
0
6
5
4
7
6
5
4
7
6
5
4
7
6
5
4
7
6
5
7
6
5
7
6
5
7
6
5
7
8
6
7
8
6
7
8
6
9
7
8
6
9
7
8
6
9
One
7
10
8
6
9
One
7
10
8
6
9
One
11
7
10
8
6
9
One
11
7
10
8
6
9
One
11
13
12
7
10
8
6
9
One
11
13
12
7
10
8
6
9
One
11
13
12
7
10
8
6
9
One
11
13
12
7
One.
One.
2 books
One.
ONE