C :: Calculate Value Of Math Expression Using Strings

Here is my objective: Write a c program that calculates the value of a mathematical expression comprised of positive numbers and the operations "+" and "-" . Specifically, first prompt the user to input an expression, read itin as a string, and then print the value of the expression. You may assume that the expression does not contain spaces, maximumsize of the expression (including digits and operators) can be 20,and that all numbers are single digit numbers.

Note that, the digits would be read in as characters; you will need to translate them to numbers (recall the ASCII table). Implementation Requirements:

Write a function called "evaluate" that takes as input a mathematical expression(as a string) and returns the value of the expression. The prototype of the function is: intevaluate(char expr[]);

Sample Output:
Input: 4+2-1+7 Output:12

Code:
int main()
{
char expr[21];
int a,ssum;
printf("Input: ");
scanf("%20s", expr);

[Code] .....

the program runs, but the output is not coming out correct.