Sum up counter values using custom sensor?

Hi. In my electric vehicle, I have an energy meter which gives one "S0" pulse for every WattHour charged. The accumulated values for the last minute are reported to the Wialon server every minute, using the IPS protocol. It can look like this:

17:09:14  36
17:10:14  35
17:11:13  35
17:12:14  36
...

How can we sum up these individual values to a total accumulated value? Ie, we would like to create a sensor which returns:

17:09:14  36
17:10:14  71
17:11:13  106
17:12:14  142
...

Is this at all possible? I've tried creating a "Counter Sensor", but neither Instant nor Differential works as expected. The just return the same (original) value.