A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 1806816 can be evenly divided by 1 2 3 4 6 8 11 12 16 22 24 29 32 33 44 48 58 59 66 87 88 96 116 118 132 174 176 177 232 236 264 319 348 352 354 464 472 528 638 649 696 708 928 944 957 1,056 1,276 1,298 1,392 1,416 1,711 1,888 1,914 1,947 2,552 2,596 2,784 2,832 3,422 3,828 3,894 5,104 5,133 5,192 5,664 6,844 7,656 7,788 10,208 10,266 10,384 13,688 15,312 15,576 18,821 20,532 20,768 27,376 30,624 31,152 37,642 41,064 54,752 56,463 62,304 75,284 82,128 112,926 150,568 164,256 225,852 301,136 451,704 602,272 903,408 and 1,806,816, with no remainder.
Since 1,806,816 cannot be divided by just 1 and 1,806,816, it is not a prime number.