integer function F (n)

    integer n

    <...>
    if (n .eq. 0) then

        F = 0

        return

    end if

    <...>

    do 100, i = 2, n, 1

        <...>

100 continue

    F = s

    return

  end

public static int F (int n) {
    int s,t,i,tmp;

    if (n==0) return(0);

    s = 1; t = 1;

    for (i=2; i<=n; i++) {

        tmp = s + t;

        s = t; t = tmp;
    }

    return(s);

}