Please read the following table in Markdown format and then answer the question according to the table. Table cells in one row are seperated by '|', and different rows are seperated by '
'.

Table:
| Pennant | Name | Hull builder | Ordered | Laid down | Launched | Accepted into service | Commissioned | Est. building cost | Fate |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
| F169 | Amazon | Vosper Thornycroft, Woolston | 26 March 1969 | 6 November 1969 | 26 April 1971 | 19 July 1974 | 11 May 1974 | £16.8M | To Pakistan as Babur |
| F170 | Antelope | Vosper Thornycroft | 11 May 1970 | 23 March 1971 | 16 March 1972 | 30 June 1975 | 16 July 1975 | £14.4M | Bombed by Argentine A-4 Skyhawks on 23 May 1982 and sank following day in San Carlos Water |
| F172 | Ambuscade | Yarrow Shipbuilders, Scotstoun | 11 November 1971 | 1 September 1971 | 18 January 1973 | 23 August 1975 | 5 September 1975 | £16.5M | To Pakistan as Tariq |
| F173 | Arrow | YSL | 11 November 1971 | 28 September 1972 | 5 February 1974 | 16 May 1975 | 29 July 1976 | £20.2M | To Pakistan as Khaibar |
| F171 | Active | Vosper Thornycroft | 11 May 1970 | 21 July 1971 | 23 November 1972 | 2 June 1977 | 17 June 1977 | £24.1M | To Pakistan as Shah Jahan |
| F174 | Alacrity | YSL | 11 November 1971 | 5 March 1973 | 18 September 1974 | 2 April 1977 | 2 July 1977 | £23.8M | To Pakistan as Badr |
| F184 | Ardent | YSL | 11 November 1971 | 26 February 1974 | 9 May 1975 | 10 September 1977 | 14 October 1977 | £26.3M | Bombed by Argentine A-4 Skyhawks on 21 May 1982 in San Carlos Water and sank following day in Grantham Sound |
| F185 | Avenger | YSL | 11 November 1971 | 30 October 1974 | 20 November 1975 | 15 April 1978 | 15 April 1978 | £27.7M | To Pakistan as Tippu Sultan |
Question:
how many boats costed less than £20m to build?
Answer:
3