CF124A The number of positions

Petr stands in line of $ n $ people, but he doesn't know exactly which position he occupies. He can say that there are no less than $ a $ people standing in front of him and no more than $ b $ people standing behind him. Find the number of different positions Petr can occupy.

The only line contains three integers $ n $ , $ a $ and $ b $ ( $ 0

Print the single number — the number of the sought positions.

The possible positions in the first sample are: 2 and 3 (if we number the positions starting with 1). In the second sample they are 3, 4 and 5.