Consider the following code:
In[4]:= Solve[Sum[i, {i, 1, n}] == 253, n]
Out[4]= {{n -> -23}, {n -> 22}}
In[7]:= Solve[\!\(
\*UnderoverscriptBox[\(\[Sum]\), \(i = 1\), \(x\)] i\) == 253, x]
Out[7]= {{x -> -23}, {x -> 22}}
In[9]:= \!\(
\*UnderoverscriptBox[\(\[Sum]\), \(i = 1\), \(22\)]i\)
Out[9]= 253