You can download a compiler here.
To compile the program type this into your console.
f90 -o F90Calculator F90Calculator.f90
After executing the calculator you can enter the first number.
Enter first number:
5 # 5 is the input
Then choose between the calculation functions.
Choose between:
1) Addition
2) Subtraction
3) Multiplication
4) Division
1 # < This is our input
We now can enter the second number
Enter second number:
2 # 2 is the input
Now it should give the right output.
6.9999999E-07 # this is the return value