sum = 100 + 102 + 104 + ... + 296 + 298 + 300.
N = (300-100)/2 +1 = 101 - the number of integers in the sum
Now, you can add first and last integers (100+300=400), then second and 100th integers (102+298=400) and so on.
Finally, we will have 50 times 400 and one integers at the middle (200) as we have odd number of integers.
So, sum = 50 * 400 + 200 = 20200
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