New Post: Get Pre-Last Item in SP2010 List

Hi All please help!
i need to calculate the value of the field, adding to it the previous value of the same field.
i have SharePoint 2010 List with 2 field (Field1/number, Field2/date).
users via table post in this list 30-31 items per one time (Field2 = day on the currents month 01/12/2013, 02/12/2013....31/12/2013).
i set button on the page and want calculate value in Field1 like:
1 item: Field1='0'; //0
2 item: Field1="1item Field1"+Field1; //0+Current Field1 value
3 item: Field1="2item Field1"+Field1;
4 item: Field1="3item Field1"+Field1;
i cant use "Last ID-1"-method because users can remove items before calculate it.
may be use Field2? Last Date - 1 day?
Please help can i use SPUpdateMultipleListItems for it? How as one click recalculate all 30 items.
Sorry for my English...