 <p>Ok then the correct source code is </p> <pre><code>program operateur implicit none CHARACTER(LEN=1) :: oper real::a,b,res print*,'Give the first number a :' read*,a print*,'Give the second number b :' read*,b print*,'which operation ?' read (*,"(A1)") oper select case (oper) case ('+') res=a+b case ('-') res=a-b case ('*') res=a*b case ('/') res=a/b case default print*, "Invalid Operator, thanks" end select print*,'the result is ',res end program operateur </code></pre>