1 + 3 + 5 + ... + 19 =
[ ( 19 - 1 ) : 2 + 1 ] × [ ( 1 + 19 ) : 2 ] =
( 18 : 2 + 1 ) × ( 20 : 2 ) =
( 9 + 1 ) × 10 =
10 × 10 = 100
