Write a program to calculate Euler's constant **e** using a series in
which terms of the form 1/n! are added to a variable **sum** in a **for j
in range()** loop. The value n! is not calculated using a function; it is
evaluated by multiplying the variable **prod** by **j** for each value
of **j**. The initial value of **sum** is one and that of **prod** is
also one.