not able to understand the output in implementing synchronization of resources in threads

See more:

C#3.0

.NET3.5

C#

.NET

I wrote the code

class Math
{
public int value1;
public int value2;
private int result;
public Math(int _value1, int _value2)
{
value1 = _value1;
value2 = _value2;
}
public void Add()
{
result = value1 + value2;
Thread.Sleep(1000);
Console.WriteLine("Add: " + result);
}
public void Subtract()
{
result = value1 - value2;
Thread.Sleep(1000);
Console.WriteLine("Subtract: " + result);
}
public void Multiply()
{
result = value1 * value2;
Thread.Sleep(1000);
Console.WriteLine("Multiply: " + result);
}
}

when i run this
everytime i am getting different output ...

Add: 6
Subtract: 6
Multiply: 6

when i run again application i get

Add: -1
Subtract: -1
Multiply:-1 

I am not able to understand the output
regards