Strong Numbers

An integer is called strong if it's decimal representation a 1 a 2 a 3 a 4 \overline { { a }_{ 1 }{ a }_{ 2 }{ a }_{ 3 }{ a }_{ 4 } } satisfies a i < a i + 1 { a }_{ i }<{ a }_{ i+1 } if a i a_i is odd and a i > a i + 1 { a }_{ i }>{ a }_{ i+1 } if a i a_i is even.

How many 4 digit strong numbers exist?

Bonus: how many 4 digit strong numbers have distinct digits?

Note: a 1 a 2 a 3 a 4 \overline { { a }_{ 1 }{ a }_{ 2 }{ a }_{ 3 }{ a }_{ 4 } } denotes concatenation and a 1 a 2 a 3 a 4 = 1000 a 1 + 100 a 2 + 10 a 3 + a 4 \overline { { a }_{ 1 }{ a }_{ 2 }{ a }_{ 3 }{ a }_{ 4 } } = 1000{ a }_{ 1 } + 100{ a }_{ 2 }+10{ a }_{ 3 }+{ a }_{ 4 }


The answer is 640.

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