A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 2181816 can be evenly divided by 1 2 3 4 6 7 8 9 12 13 14 18 21 24 26 27 28 36 37 39 42 52 54 56 63 72 74 78 81 84 91 104 108 111 117 126 148 156 162 168 182 189 216 222 234 252 259 273 296 312 324 333 351 364 378 444 468 481 504 518 546 567 648 666 702 728 756 777 819 888 936 962 999 1,036 1,053 1,092 1,134 1,332 1,404 1,443 1,512 1,554 1,638 1,924 1,998 2,072 2,106 2,184 2,268 2,331 2,457 2,664 2,808 2,886 2,997 3,108 3,276 3,367 3,848 3,996 4,212 4,329 4,536 4,662 4,914 5,772 5,994 6,216 6,552 6,734 6,993 7,371 7,992 8,424 8,658 9,324 9,828 10,101 11,544 11,988 12,987 13,468 13,986 14,742 17,316 18,648 19,656 20,202 20,979 23,976 25,974 26,936 27,972 29,484 30,303 34,632 38,961 40,404 41,958 51,948 55,944 58,968 60,606 77,922 80,808 83,916 90,909 103,896 121,212 155,844 167,832 181,818 242,424 272,727 311,688 363,636 545,454 727,272 1,090,908 and 2,181,816, with no remainder.
Since 2,181,816 cannot be divided by just 1 and 2,181,816, it is not a prime number.