### Instruction: What do you need when you see a blue flag during the race?
### Input: Race
The race begins with a warm-up lap, after which the cars assemble on the starting grid in the order they qualified. This lap is often referred to as the formation lap, as the cars lap in formation with no overtaking (although a driver who makes a mistake may regain lost ground). The warm-up lap allows drivers to check the condition of the track and their car, gives the tyres a chance to warm up to increase traction and grip, and also gives the pit crews time to clear themselves and their equipment from the grid for the race start.


Jacques Villeneuve qualifying at the 2005 United States Grand Prix in his Sauber C24
Once all the cars have formed on the grid, after the medical car positions itself behind the pack, a light system above the track indicates the start of the race: five red lights are illuminated at intervals of one second; they are all then extinguished simultaneously after an unspecified time (typically less than 3 seconds) to signal the start of the race. The start procedure may be abandoned if a driver stalls on the grid or on the track in an unsafe position, signalled by raising their arm. If this happens, the procedure restarts: a new formation lap begins with the offending car removed from the grid. The race may also be restarted in the event of a serious accident or dangerous conditions, with the original start voided. The race may be started from behind the Safety Car if race control feels a racing start would be excessively dangerous, such as extremely heavy rainfall. As of the 2019 season, there will always be a standing restart. If due to heavy rainfall a start behind the safety car is necessary, then after the track has dried sufficiently, drivers will form up for a standing start. There is no formation lap when races start behind the Safety Car.

Under normal circumstances, the winner of the race is the first driver to cross the finish line having completed a set number of laps. Race officials may end the race early (putting out a red flag) due to unsafe conditions such as extreme rainfall, and it must finish within two hours, although races are only likely to last this long in the case of extreme weather or if the safety car is deployed during the race. When a situation justifies pausing the race without terminating it, the red flag is deployed; since 2005, a ten-minute warning is given before the race is resumed behind the safety car, which leads the field for a lap before it returns to the pit lane (before then the race resumed in race order from the penultimate lap before the red flag was shown).

In the 1950s, race distances varied from 300 km (190 mi) to 600 km (370 mi). The maximum race length was reduced to 400 km (250 mi) in 1966 and 325 km (202 mi) in 1971. The race length was standardised to the current 305 km (190 mi) in 1989. However, street races like Monaco have shorter distances, to keep under the two-hour limit.

Drivers may overtake one another for position over the course of the race. If a leader comes across a backmarker (slower car) who has completed fewer laps, the back marker is shown a blue flag telling them that they are obliged to allow the leader to overtake them. The slower car is said to be "lapped" and, once the leader finishes the race, is classified as finishing the race "one lap down". A driver can be lapped numerous times, by any car in front of them. A driver who fails to complete more than 90% of the race distance is shown as "not classified" in the results.

Throughout the race, drivers may make pit stops to change tyres and repair damage (from 1994 to 2009 inclusive, they could also refuel). Different teams and drivers employ different pit stop strategies in order to maximise their car's potential. Three dry tyre compounds, with different durability and adhesion characteristics, are available to drivers. Over the course of a race, drivers must use two of the three available compounds. The different compounds have different levels of performance and choosing when to use which compound is a key tactical decision to make. Different tyres have different colours on their sidewalls; this allows spectators to understand the strategies.

Under wet conditions, drivers may switch to one of two specialised wet weather tyres with additional grooves (one "intermediate", for mild wet conditions, such as after recent rain, one "full wet", for racing in or immediately after rain). A driver must make at least one stop to use two tyre compounds; up to three stops are typically made, although further stops may be necessary to fix damage or if weather conditions change. If rain tyres are used, drivers are no longer obliged to use both types of dry tyres.
### Response: Allow the leader to overtake.