def f2(x, y, z):
arg = x, y, z
try:
return sum((i if isinstance(i, int)\
else int(i) if isinstance(i, str) and int(i) >= 0\
else None for i in arg))
except:
return None
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assert f2(1, 2, 3) == 6
assert f2(5, "3", "0") == 8
assert f2("2", -5, "3") == 0
assert f2("a", 4, "0") == None
assert f2("2", 3, "-5") == None
assert f2(3, 1, 3)
assert f2(3, "1", 3)