How does Gauss calculate "1 + 2 + 3 +. + 97 + 98 + 99 + 100 = 5050"

How does Gauss calculate "1 + 2 + 3 +. + 97 + 98 + 99 + 100 = 5050"

1+2+3+ . +97+98+99+100
=(1+100)+(2+99)+.+(50+51)
=101+101+.+101
There are 50 groups
So,
Original formula = 101 × 50
=5050