Subtracting from file, outputting calculation (c++) - c++

Question

Lets say I have a text file that contains the value 1000 and the user inputs how much they want to subtract from the 1000, as many times as they like. Is there any way to output the actual calculation?
int read_balance(void);
void write_balance(int balance);
using namespace std;
int _tmain(int argc, _TCHAR* argv[])
{
int selection;
int total;
int attempts = 0;
string name;
string number;
int amount = 0;
do {
cout << "1. Transfer an amount" <<endl;
cout << "2. List recent transactions"<<endl;
cout << "3. Display account details and current balance"<<endl;
cout << "4. Quit" << endl;
cout << "Please enter menu number"<<endl;
cout << "" << endl;
cin >> selection;
int amount = 0;
switch(selection)
{
case 1:
cout << "You have choosen to transfer an amount" << endl;
cout << "How much do you wish to transfer from the shop account?"<<endl;
cout << "transfer funds" << endl;
if (std::cin >> amount)
{
std::cout << "Transferred Amount:" << amount << "\n";
int balance = read_balance();
if (amount <= 0)
{
std::cout << "Amount must be positive\n";
}
else if (balance < amount)
{
std::cout << "Insufficient funds\n";
}
else
{
int new_balance = balance - amount;
write_balance(new_balance);
std::cout << "New account balance: " << new_balance << std::endl;
fstream infile("time.txt", ios::app);
std::time_t result = std::time(nullptr);
std::string timeresult = std::ctime(&result);
infile << amount << std::endl;
infile << timeresult << std::endl;
}
}
break;
case 2:
cout << "Here are you're recent transactions" <<endl;
cout << "" << endl;
cout << "" << endl;
break;
case 3:
cout << "The account names is:" << endl;
cout << "The account number is:" << endl;
std::cout << "The current account balance is " << read_balance() << std::endl;
break;
case 4:
return 0;
break;
default:
cout << "Ooops, invalid selection!" << endl;
break;
}
}while(selection != 4);
system("pause");
return 0;
}
int read_balance(void)
{
std::ifstream f;
f.exceptions(std::ios::failbit | std::ios::badbit);
f.open("shop.txt");
int balance;
f >> balance;
f.close();
return balance;
}
void write_balance(int balance)
{
std::ofstream f;
f.exceptions(std::ios::failbit | std::ios::badbit);
f.open("shop.txt");
f << balance;
f.close();
}