The numbers that can be arranged to form a rectangle are called Rectangular Numbers (also known as Pronic numbers). The first few rectangular numbers are:
0, 2, 6, 12, 20, 30, 42, 56, 72, 90, 110, 132, 156, 182, 210, 240, 272, 306, 342, 380, 420, 462 . . . . . .
Given a number n, find n-th rectangular number.
Input : 1 Output : 2 Input : 4 Output : 20 Input : 5 Output : 30
The number 2 is a rectangular number because it is 1 row by 2 columns. The number 6 is a rectangular number because it is 2 rows by 3 columns, and the number 12 is a rectangular number because it is 3 rows by 4 columns.
If we observe these numbers carefully, we can notice that n-th rectangular number is n(n+1).
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Improved By : jit_t