void frac_input(int &num1, int &den1, int &num2, int &den2, char &oper){
	char trash;
	bool valid;
	do{
		cout << "Enter like this: 4 / 2 + 3 / 3: ";
		cin >>num1>>trash>>den1>>oper>>num2>>trash>>den2;
		if(den1 == 0 || den2 == 0)
			valid = false;
		if(!((isdigit(num1) && isdigit(num2)) && (isdigit(den1) && isdigit(den2))))
			valid = false;
		if(oper != '*' && oper != '/' && oper != '+' && oper != '-')
			valid = false;
		if(valid != false)
			valid = true;
	} while (valid = false);
}