For some reasons, with the simple function to find the factorial in Dart language. It does not work right

Total: 1
Next: 1
Total: 1
Next: 2
Total: 2
Next: 3
Total: 6
Next: 4
Total: 24
Next: 5
Total: 120
Next: 6
Total: 720
Next: 7
Total: 5040
Next: 8
Total: 40320
Next: 9
Total: 362880
Next: 10
Total: 3628800
Next: 11
Total: 39916800
Next: 12
Total: 479001600
Next: 13
Total: 6227020800
Next: 14
Total: 87178291200
Next: 15
Total: 1307674368000
Next: 16
Total: 20922789888000
Next: 17
Total: 355687428096000
Next: 18
Total: 6402373705728000
Next: 19
Total: 121645100408832000
Next: 20
Total: 2432902008176640000
Next: 21
Total: -4249290049419214848
Next: 22
Total: -1250660718674968576
Next: 23
Total: 8128291617894825984
Next: 24
Total: -7835185981329244160
Next: 25

In the step
Total: 2432902008176640000
Next: 21
Total: -4249290049419214848
It should be take the total and multiple to 21 = 5.1090942e+19 but the result was negative number
I have tried in Dartpad and the result is correct

Can anyone explain what the hell is going here ?????? PLEASE !!!

Your code cant check if the number is 5xx, x5x,...These numbers include 5 digit too

OMG ! I laugh like crazy

OMG ! I laugh like crazy