A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 1860408 can be evenly divided by 1 2 3 4 6 8 9 11 12 18 22 24 27 29 33 36 44 54 58 66 72 81 87 88 99 108 116 132 162 174 198 216 232 243 261 264 297 319 324 348 396 486 522 594 638 648 696 729 783 792 891 957 972 1,044 1,188 1,276 1,458 1,566 1,782 1,914 1,944 2,088 2,349 2,376 2,552 2,673 2,871 2,916 3,132 3,564 3,828 4,698 5,346 5,742 5,832 6,264 7,047 7,128 7,656 8,019 8,613 9,396 10,692 11,484 14,094 16,038 17,226 18,792 21,141 21,384 22,968 25,839 28,188 32,076 34,452 42,282 51,678 56,376 64,152 68,904 77,517 84,564 103,356 155,034 169,128 206,712 232,551 310,068 465,102 620,136 930,204 and 1,860,408, with no remainder.
Since 1,860,408 cannot be divided by just 1 and 1,860,408, it is not a prime number.