A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 1878660 can be evenly divided by 1 2 3 4 5 6 7 9 10 12 14 15 18 20 21 27 28 30 35 36 42 45 49 54 60 63 70 71 84 90 98 105 108 126 135 140 142 147 180 189 196 210 213 245 252 270 284 294 315 355 378 420 426 441 490 497 540 588 630 639 710 735 756 852 882 945 980 994 1,065 1,260 1,278 1,323 1,420 1,470 1,491 1,764 1,890 1,917 1,988 2,130 2,205 2,485 2,556 2,646 2,940 2,982 3,195 3,479 3,780 3,834 4,260 4,410 4,473 4,970 5,292 5,964 6,390 6,615 6,958 7,455 7,668 8,820 8,946 9,585 9,940 10,437 12,780 13,230 13,419 13,916 14,910 17,395 17,892 19,170 20,874 22,365 26,460 26,838 29,820 31,311 34,790 38,340 41,748 44,730 52,185 53,676 62,622 67,095 69,580 89,460 93,933 104,370 125,244 134,190 156,555 187,866 208,740 268,380 313,110 375,732 469,665 626,220 939,330 and 1,878,660, with no remainder.
Since 1,878,660 cannot be divided by just 1 and 1,878,660, it is not a prime number.