i need to use if condition

Question

Purchase
Sales
Return
Balance

in my TextBox4_TextChanged it throw error as

Server Error in '/retrievedata' Application.

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Compilation Error

Description: An error occurred during the compilation of a resource required to service this request. Please review the following specific error details and modify your source code appropriately. 

Compiler Error Message: CS0103: The name 'c' does not exist in the current context

Source Error:

Line 85:         } Line 86:  Line 87:         else if (c != 0) Line 88:         { Line 89:             int c = int.Parse(TextBox3.Text) + int.Parse(TextBox4.Text);

Source File: d:\sample tools\retrievedata\Default.aspx.cs    Line: 87 

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i need in the balnce field as output as 5.in the textbox