[Python] Adding previous rows to generate another row in pandas

I am trying to solve a problem in my data-frame

df.head()
0 Key value
1 10 500
2 11 500
3 12 600
4 12 800
5 13 1000
6 13 1200
.
.
.
200++


output is to put the values in the above data-frame or have another data-frame with all the values of above with additional info show as below.

Expected Output:

0 Key value
1 10 500
2 11 500
3 12 600
4 12 800
5 12 1400 -----> Addition of 800+600 as keys are same
6 13 1000
7 13 1200
8 13 2200 -----> Addition of 1000+12000 as keys are same
.
.
200++


I am just starting out in Pandas, any help will be appreciate.

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