It is stipulated that 3△2=3+33=36, 2△3=2+22+222=246, 1△4=1+11+111+1111=1234, then what is 6△7 equal to?
规定3△2=3+33=36，2△3=2+22+222=246，1△4=1+11+111+1111=1234，那么6△7等于多少．
<gadget id="calculator">6 + 66 + 666 + 6_666 + 66_666 + 666_666 + 6_666_666</gadget>
<output>7_407_402</output>

<result>7_407_402</result>