Using a windowed counter

Hi,

I am a newcomer, so I am probably asking something silly here.

So, I want to use rrdtool to manage my weather station data. I read data in 5 minute intervals, but my station only tells me the rain counter for the last hour. So the number I get is a summation of my rain counter deltas for the last 12 intervals. 

For instance, suppose in the last 5 hours I get the values the first line below (one char for each 5-min interval), it means that, for every 5 minutes, actual rainfall has been as in the second line.

011111244445678888889998767777765555553444335888889899987677
010000120001121000121000012000110000010100012300011010001210

Is there a way to specify that the values to store in the RRD would be calculated as:

stored[now] = input[now] - input[now - 1] + stored[now - 12]

Thanks a lot!

MP