How do I subtract two large hexadecimal numbers?

I have two large hexadecimal numbers (two hex strings of 128 characters each), I need to subtract one from the other, how do I do it in c++?

std::string hex1 = "77733fc18367eaf818316159595cbb1e1fa8443efb8e9592d0f70a6301776b59f2f8e9d895a5f932cc363465cd10bfc34eac937825aa755ae1388b36b68d4705";
std::string hex2 = "79be667ef9dcbbac55a06295ce870b07029bfcdb2dce28d959f2815b16f81798483ada7726a3c4655da4fbfc0e1108a8fd17b448a68554199c47d08ffb10d4b8";
//for example:
std::string result = hex2 - hex1; //how to do it?