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for the left most digit, if it takes on the value of 4, it will be 4x10^3,
but this can appear 4^3 times, since there are 3 other digits undecided.
So we get (4^3)x4x10^3, similiarly for 1,2,3
the total will be (4^3)x(4+3+2+1)x10^3
repeat the same for the other three digits, we get:
1111x640 = 711040