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string originalString = "Hello, world!";
int index = 5;
string firstPart = originalString.Substring(0, index); // "Hello"
string secondPart = originalString.Substring(index); // ", world!"
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string a = input.Substring(0, 10);
string b = input.Substring(10, 5);
string c = input.Substring(15, 3);