Detailed Instructions: This task involves annotating the answer type to a given question that involve some kind of complex reasoning (including numerical reasoning). Note that the questions require looking at more than one part of the passage to answer. There are 3 possible answer types (i) spans, (ii) numbers and (iii) dates. If the answer can be found in the passage, label it as "span". If the answer is a number, label as "number". Similarly, label "date" if you think the answer to the given question is a date.
Q: Passage: At the beginning of the period the Time of Troubles disorganized the frontier defenses and led to much destruction. The Oka region was raided almost every year, but with decreasing frequency. 1633 may have been the last crossing of the Oka. We hear more frequently of raids at such places as Kursk and Voronezh, implying stronger defense in the north and a southern movement of the Russian population. The Belgorod Line of forts was begun about 1633 and seems to have had noticeable effects after about 1640. We hear of raids on Poland in 1614-21, 1624-28, 1633, 1640 and 1642.
Question: How many years since the first and last raid passed that we heard about in Poland?
A:
number