ohhhh so that's it

Here is a simple example for which your function returns an incorrect result:

intervals = {{1, 4}, {3, 5}, {4, 6}};

The expected result is 5 but your function returns 6.

I hate that I used namespace std for no reason cuz i was lazy to add it in vscode-_-

dude i even commentede it in my code lmao xD
/damn I didn't read that part xD{"The sum of the lengths of these intervals is 7.
Since [1, 4] and [3, 5] overlap, we can treat the interval as [1, 5],
which has a length of 4."}/

was gonna sort and check but that was too slow xD
so made this xD

never thought I would see asm here

don't worry bro
there is always walls to climb.
just work on your self and you will get better in no time

a good way to start is
try suming a few rows maybe u find something

dued it took me 10 seconds to get it
there is no lack of pocurrent nor lack of explanation
just think a bit or search the math behind it

honestly you could say you opened my eyes. xD
my ansewr is way simpler compared to this xD

don't ask me why

also auto is a way to let the compiler detrmine the var trype by itself

its a ranged loop