A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 820440 can be evenly divided by 1 2 3 4 5 6 8 9 10 12 15 18 20 24 30 36 40 43 45 53 60 72 86 90 106 120 129 159 172 180 212 215 258 265 318 344 360 387 424 430 477 516 530 636 645 774 795 860 954 1,032 1,060 1,272 1,290 1,548 1,590 1,720 1,908 1,935 2,120 2,279 2,385 2,580 3,096 3,180 3,816 3,870 4,558 4,770 5,160 6,360 6,837 7,740 9,116 9,540 11,395 13,674 15,480 18,232 19,080 20,511 22,790 27,348 34,185 41,022 45,580 54,696 68,370 82,044 91,160 102,555 136,740 164,088 205,110 273,480 410,220 and 820,440, with no remainder.
Since 820,440 cannot be divided by just 1 and 820,440, it is not a prime number.