z = sscanf(str, s, &y, &x);
          if (z == 2) goto L1
          explode();
L1:
          t1 = y;
          t2 = 2 * t1;
          t1 = x;
          if (t2 == t1) goto L2;
          explode();
L2:

Note: s must be the address of the format string.

It appears that the format string must have 2 conversion specifications, but what are they?