Hi Maxim, You need to explicitly make the constant an unsigned long long. You do not get implicit conversion from a constant to a unsigned long long. Why? Well, because unsigned long long is not in the standard, it's an extension. So try this: l1: unsigned long long a; l2: cout << sizeof(a) << endl; l3: a = 1281474976710655ULL; l4: cout << a << endl; HTH, --Eljay