"""Parse interval string or integer into seconds.

This function converts an interval, which can be specified as an integer (representing seconds) or a string (with a number and a unit), into the total number of seconds. The string format should consist of a number followed by a unit (e.g., "1h", "30m", "7d"). If no unit is provided in the string, it is assumed to be in seconds.

Args:
    interval: Can be:
        - Integer: seconds (e.g., 3600)
        - String: number + unit (e.g., "1h", "30m", "7d")

Returns:
    int: Total seconds

Raises:
    ValueError: If the interval string is in an invalid format or contains an unsupported unit.

Examples:
    parse_interval(3600) -> 3600
    parse_interval("1h") -> 3600
    parse_interval("30m") -> 1800
    parse_interval("7d") -> 604800
"""
You should write code starting with:
def parse_interval(interval) -> int:
    pass

