#define if_c(x) (x) ?
#define else_c :

constexpr int64_t muldiv(int64_t a, int64_t b, int64_t c, unsigned n = 0) {
  return
    if_c(a < 0x7FFFFFFF && b < 0x7FFFFFFF)
      (a * b) / c
    else_c
      if_c(n != 2)
        if_c(c <= a)
          ((a / c) * b + muldiv(b, a % c, c, n + 1))
        else_c
          muldiv(a, b, c / 2) / 2
     else_c 0;
}