Include `using System.Numerics`

and add a reference to `System.Numerics`

to the project.

```
using System;
using System.Numerics;
namespace Euler_25
{
class Program
{
static void Main(string[] args)
{
BigInteger l1 = 1;
BigInteger l2 = 1;
BigInteger current = l1 + l2;
while (current.ToString().Length < 1000)
{
l2 = l1;
l1 = current;
current = l1 + l2;
}
Console.WriteLine(current);
}
}
}
```

This simple algorithm iterates through Fibonacci numbers until it reaches one at least 1000 decimal digits in length, then prints it out. This value is significantly larger than even a `ulong`

could hold.

Theoretically, the only limit on the `BigInteger`

class is the amount of RAM your application can consume.

Note: `BigInteger`

is only available in .NET 4.0 and higher.