This is an example of how to declare a __int64 variable and print it. Remember while printing its not %d, its %I64d. Just remember this while printing.
__int64 i = -1234567890123456789; // a negative number.
You can use unsigned for another extra bit to extend. But you can't use any negative number in this case. Also remember this %I64u to denote the unsigned number.
unsigned __int64 i = 1234567890123456789;
If you are using gcc compiler, then you have to use long long as the variable name and while printing, you have to use it %lld instead of %I64d.
long long i = -1234567890123456789; // a negative number.
Similar for unsigned long long below example with %llu when needs to print.
unsigned long long i = 1234567890123456789;
Mainly these types of variable commonly used by the programmer of ACM programming contest. But when sometimes it requires another more large number, like few thousands digits number, that cases the use their custom defined number class that can handle arithmetic operation with very long number.