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:
| Driver | Constructor | Laps | Time/Retired | Grid |
| --- | --- | --- | --- | --- |
| Mika Häkkinen | McLaren - Mercedes | 71.0 | 1:30:44.086 | 3.0 |
| David Coulthard | McLaren - Mercedes | 71.0 | +5.289 | 14.0 |
| Michael Schumacher | Ferrari | 71.0 | +39.092 | 4.0 |
| Eddie Irvine | Ferrari | 71.0 | +43.976 | 8.0 |
| Ralf Schumacher | Jordan - Mugen-Honda | 71.0 | +50.654 | 9.0 |
| Jacques Villeneuve | Williams - Mecachrome | 71.0 | +53.202 | 11.0 |
| Damon Hill | Jordan - Mugen-Honda | 71.0 | +1:13.624 | 15.0 |
| Johnny Herbert | Sauber - Petronas | 70.0 | +1 lap | 18.0 |
| Alexander Wurz | Benetton - Playlife | 70.0 | +1 lap | 17.0 |
| Jarno Trulli | Prost - Peugeot | 70.0 | +1 lap | 16.0 |
| Shinji Nakano | Minardi - Ford | 70.0 | +1 lap | 21.0 |
| Ricardo Rosset | Tyrrell - Ford | 69.0 | +2 laps | 22.0 |
| Jos Verstappen | Stewart - Ford | 51.0 | Engine | 12.0 |
| Esteban Tuero | Minardi - Ford | 30.0 | Spun off | 19.0 |
| Giancarlo Fisichella | Benetton - Playlife | 21.0 | Collision | 1.0 |
| Jean Alesi | Sauber - Petronas | 21.0 | Collision | 2.0 |
| Heinz-Harald Frentzen | Williams - Mecachrome | 16.0 | Engine | 7.0 |
| Rubens Barrichello | Stewart - Ford | 8.0 | Brakes | 5.0 |
| Pedro Diniz | Arrows | 3.0 | Collision | 13.0 |
| Mika Salo | Arrows | 1.0 | Collision | 6.0 |
| Olivier Panis | Prost - Peugeot | 0.0 | Clutch | 10.0 |
| Toranosuke Takagi | Tyrrell - Ford | 0.0 | Collision | 20.0 |
Question:
What is the average grid for johnny herbert with a Time/Retired of +1 lap?
Answer:
18.0