Simplify

$1-2=-1$

$2-3=-1$

$3-4=-1$ ........

every time there is -1 which has come 25 times .

so,the total calculation will be= $-1 \times 25+1=-1+1=0$ .......[that's it]

lets group this up!

we get (1-2)+(3-4)+....+(49-50)

this sum will be equal to -25

(-1 is grouped 25 times)

then, the second part..

we get (49-48)+(47-46)+....+(2-1) [although 2-1 isnt listed, it is a part of the ' .... ']

which would be 24

(1 is grouped 24 times)

then there is a '+1'

add everything and you get

-25+24+1

= $\boxed0$