Decrements the number stored at key by decrement.
If the key does not exist, it is set to 0 before performing the operation.
An error is returned if the key contains a value of the wrong type or contains a
string that can not be represented as integer.
This operation is limited to 64 bit signed integers.
See INCR for extra information on increment/decrement operations.
@return
@integer-reply: the value of key after the decrement
@examples
SET mykey "10"
DECRBY mykey 3