1) Consider the function dayOfWeek(), whose C99 prototype follows: int dayOfWeek(int day, int month, int year, int firstJan); The function takes four integer parameters, each providing information about a date and its year. The goal of the function is to firstly ensure that the provided date is valid and, if so, to determine on which day of the week the date falls. The first parameter, day, provides the day of interest its values must range from 1 to 31 (inclusive). The second parameter, month, provides the month of interest its values must range from 1 to 12 (inclusive). The third parameter, year, provides the year of interest any integer value of 1970 or greater is permitted. The final parameter, firstJan, indicates the day of the week on which the first of January falls in the provided year. Its permitted values are 0 (representing Sunday), 1 (representing Monday), and so on, up to 6 (representing Saturday). If all provided parameters are valid, the function will return the day of the week on which the indicated date falls. For example, the call dayOfWeek(13, 11, 2017, 0); will return the integer 1 (representing Monday). If the provided date is invalid, the function should return the integer -1. Write the function dayOfWeek() in C99.