easy way to add numbers 1 to 100

I will ask you what is the mean for values from 1 to 100 ?this is very easy question .

the mean = Sum of the values / how many numbers then  

but now the problem is to get the Sum of the values from 1 to 100 ,to do this by easy way write half of 100 twice then read the new number. in this example 100 / 2 = 50 then the Sum of the values from 1 to 100 = 5050 

so the mean = 5050 / 100 = 50.5

example : 1 + 2 + 3 + 4 + ...... + 1000 = 500500 because 1000/2 = 500 then the sum = 500500

you can do this with 10000 , 100000 , .........................