A string of numbers, arranged by 1,1,2,2,3,3,4,4,5,5,. From the first number on the left, the 25th number is? What is the sum of their first 26 numbers?

It's 1
Listed: 1122334455677889910101
26/2=13
（1+13）*13/2=91
91*2=182

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