I'm trying to create a function that takes grouped data, represented as such: list2 = {{0, 3}, {1, 7}, {2, 6}, {3, 4}, {4, 1}} and convert it into ungrouped data like this: list2new = {0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 2, 2, 2, 2, 2, 2, 3, 3, 3, 3, 4}.

I've written a code block that produces the result I'm after:

n = 0;
string = "{";
While[n != Length[list2],
 n = n + 1;
 For[i = 0, i < list2[[n, 2]], i++,
  string = string <> ToString[list2[[n, 1]]] <> ", ";]]
string = StringDrop[string, -2] <> "}";
ToExpression[string]

However, now when I try to assign this to a variable or define it as a function it develops errors. For example this code doesn't work:

UngroupList[in_List] := (
  n = 0;
  string = "{";
  While[n != Length[in],
     n = n + 1;
     For[i = 0, i < in[[n, 2]], i++,
      string = string <> ToString[in[[n, 1]]] <> ", ";]]
    string = StringDrop[string, -2] <> "}";
  ToExpression[string])

UngroupList[list2]

Hoping someone can see what I'm doing wrong here. Thanks for your time.