The third and fourth day contributed 6+18=<<6+18=24>>24 inches to the depth of the snowdrift.
Therefore, the size of the snowdrift after half was melted on the second day, but before the snowfall on the third and fourth day, was 34-24=<<34-24=10>>10 inches.
If the size of the drift on the second day, after half of the snow had melted, was 10 inches, then the depth of the original drift created on day 1 was 2*10=20 inches deep.
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