Beginning Zeros

You have a string that consist only of digits. You need to find how many zero digits ("0") are at the beginning of the given string.

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Input:
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A string, that consist of digits.

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Output:
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An Int.

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Example:
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beginning_zeros('100') == 0 beginning_zeros('001') == 2 beginning_zeros('100100') == 0 beginning_zeros('001001') == 2 beginning_zeros('012345679') == 1 beginning_zeros('0000') == 4

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Precondition:
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0-9
*