A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 3651804 can be evenly divided by 1 2 3 4 6 9 12 13 17 18 26 27 34 36 39 51 52 54 68 78 81 102 108 117 153 156 162 204 221 234 243 289 306 324 351 442 459 468 486 578 612 663 702 867 884 918 972 1,053 1,156 1,326 1,377 1,404 1,734 1,836 1,989 2,106 2,601 2,652 2,754 3,159 3,468 3,757 3,978 4,131 4,212 5,202 5,508 5,967 6,318 7,514 7,803 7,956 8,262 10,404 11,271 11,934 12,636 15,028 15,606 16,524 17,901 22,542 23,409 23,868 31,212 33,813 35,802 45,084 46,818 53,703 67,626 70,227 71,604 93,636 101,439 107,406 135,252 140,454 202,878 214,812 280,908 304,317 405,756 608,634 912,951 1,217,268 1,825,902 and 3,651,804, with no remainder.
Since 3,651,804 cannot be divided by just 1 and 3,651,804, it is not a prime number.