Kind of funny, but I had this impression you were a brain damaged vet in re-hab, and re-learning arithmetic. <ROFL!>
Your problem is simplicity itself, *IF* your program is allowed to run.
Add in your stdio.h header and main(), and you should have a working program in jig time.
int n1, n2, sum=0;
fscanf(stdin,"%d %ch %d",&n1, &ch &n2);
sum=n1 + n2;
fprintf(stdout,"The sum is: %d\n",sum);
You may need to adjust the spacing in the fscanf() format, depending on exactly how the numbers and the + are presented.