def find_rightmost_char(s, c):
    """Takes a string s and a char (string) c. Returns the integer index of
    the reightmost occurence of c in s. If c is not in s, returns -1.
    """
    i = len(s) - 1
    while (i >= 0):
        if s[i] == c:
            break
        else: i -= 1
    return i