A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 608608 can be evenly divided by 1 2 4 7 8 11 13 14 16 19 22 26 28 32 38 44 52 56 76 77 88 91 104 112 133 143 152 154 176 182 208 209 224 247 266 286 304 308 352 364 416 418 494 532 572 608 616 728 836 988 1,001 1,064 1,144 1,232 1,456 1,463 1,672 1,729 1,976 2,002 2,128 2,288 2,464 2,717 2,912 2,926 3,344 3,458 3,952 4,004 4,256 4,576 5,434 5,852 6,688 6,916 7,904 8,008 10,868 11,704 13,832 16,016 19,019 21,736 23,408 27,664 32,032 38,038 43,472 46,816 55,328 76,076 86,944 152,152 304,304 and 608,608, with no remainder.
Since 608,608 cannot be divided by just 1 and 608,608, it is not a prime number.