5050

Number
$5050$ (five thousand and fifty) is:


 * $2 \times 5^2 \times 101$


 * The $13$th Kaprekar number after $1, 9, 45, 55, 99, 297, 703, 999, 2223, 2728, 4879, 4950$:
 * $5050^2 = 25 \, 502 \, 500 \to 2550 + 2500 = 5050$