long long in C/C++
The letters 100000000000 make up a literal integer constant, but the value is too large for the type int. You need to use a suffix to change the type of the literal, i.e.
long long num3 = 100000000000LL;
The suffix LL makes the literal into type long long. C is not "smart" enough to conclude this from the type on the left, the type is a property of the literal itself, not the context in which it is being used.
Try:
num3 = 100000000000LL;
And BTW, in C++ this is a compiler extension, the standard does not define long long, thats part of C99.