a compile problem about loop func

Hi, everyone

I'm new to fortran. From an online tutorial I've found the following
code example:

        Integer I, n, sum

        sum = 0
        do 10 i = 1, n
                  sum = sum + i
                  write(*,*) 'sum=', sum
10       continue
stop
end

When I compile this function with g77 loop.f, the error comes out:
sum = 0 ( there's a small arrow under 's')
End of source file before end of block started at (^)

Does someone know how to deal with it?

Thanks a lot