Problem With read Function

[N@R!MAN]

Here is My Code:

#include <iostream>
#include <iomanip>
#include <string>
#include <fstream>
#include <cstdlib>
using namespace std;

class Student
{
public:
	Student(string name = "", int num = 0, int unit = 0)
	{
		setName(name);
		this->num = num;
		this->unit = unit;
	}

	void set(string name = "", int num = 0, int unit = 0)
	{
		setName(name);
		this->num = num;
		this->unit = unit;
	}

	void setName(string n)
	{
		int len = n.length();
		len = (len < 15) ? len : 14;
		n.copy(this->name, len);
		name[len] = '\0';
	}

	char * getName()
	{
		return name;
	}

	int getNum()
	{
		return num;
	}

	int getUnit()
	{
		return unit;
	}

private:
	char name[15];
	int num, unit;
};

int main()
{
	ofstream o("test.bin", ios::out| ios::binary);
	ifstream f("test.bin", ios::in| ios::binary);
	Student s;
	Student st;
	Student stu;
	if(!f || !o)
	{
		cerr << "Error!";
		exit(1);
	}

	string name;
	int num, unit;

	for(int i = 0;i < 50;i++)
	{
		o.write(reinterpret_cast<const char *>(&s), sizeof(Student));
	}

	cin >> name >> num >> unit;

	while(num > 0 && num <= 50)
	{
		st.set(name, num, unit);
		o.seekp((st.getNum() - 1) * sizeof(Student));
		o.write(reinterpret_cast<const char *>(&st), sizeof(Student));
		cin >> name >> num >> unit;
	}

	f.read(reinterpret_cast<char *>(&stu), sizeof(Student));

	while(f.good())
	{
		if(stu.getNum() != 0 )
		{
			cout << "Name : " << stu.getName() << endl;
  		}

		f.read(reinterpret_cast<char *>(&stu), sizeof(Student));
	}

	f.close();

	return 0;
}

My Problem is When I Write n Student's information in file, Read Function only read n - 1 information from file! for example if I enter this input:

1 1 1
2 2 2
-1 -1 -1

Output is :
Name : 1