
Mostly just a set of sequential if statements to do the math:
#include <stdio.h>
main () {
int ans, num;
char sym[10];
FILE *fin = fopen ("carith.in", "r");
FILE *fout = fopen ("carith.out", "w");
fscanf (fin, "%d", &ans);
fscanf (fin, "%s", sym);
fscanf (fin, "%d", &num);
if (sym[0] == '+') ans += num;
if (sym[0] == '-') ans -= num;
if (sym[0] == '*') ans *= num;
if (sym[0] == '/') ans /= num;
fscanf (fin, "%s", sym);
fscanf (fin, "%d", &num);
if (sym[0] == '+') ans += num;
if (sym[0] == '-') ans -= num;
if (sym[0] == '*') ans *= num;
if (sym[0] == '/') ans /= num;
fprintf (fout, "%d\n", ans);
exit (0);
}