Starting from an array of sub-arrays, each sub-array is a 2 elements key|value pair. Example

input = [[:A, 5], [:B, 2], [:A, 12], [:B, -2], [:C, 7]]

Convert this into a hash that has distinct keys from above, with values summed up, so the above would be:

output = {:C 7, :B 0, :A 17}