How many of Dan Brown's books does Professor Robert Langdon appear in?
5
3
1
3
1
3
5
1
3
5
1
3
5
1
3
5
1
3
5
1
2
3
5
1
2
4
3
5
1
2
7
4
3
5
1
2
7
6
4
3
5
1
2
7
6
4
3
5
1
2
7
6
4
3
5
1
2
7
6
4
3
5
1
2
7
6
4
3
5
1
2
7
6
4
3
5
1
2
7
6
0
4
3
5
8
1
2
7
6
4
0
3
5
8
1
2
7
6
4
3
5
8
1
2
7
6
4
3
5
8
1
2
7
6
4
5
8
2
7
6
4
5
8
2
7
6
4
10
13
8
2
9
11
7
6
4
10
13
8
2
9
11
7
6
4
10
13
8
2
9
11
7
6
4
10
13
8
2
9
11
7
6
4
10
13
8
9
11
7
6
4
10
13
8
9
11
7
6
12
10
13
8
9
11
6
12
10
13
8
9
11
12
10
13
8
9
11
ten
14
12
ten