#include <stdio.h>
#include <stdlib.h>

int daysInMonth(int mth, int yr);
int isLeapYear(int yr);

int main(void) {

  int mth, yr;

  do{
    printf("Enter a month - a number between 1 and 12: ");
    scanf("%d", &mth);
    printf("Enter a year: ");
    scanf("%d", &yr);

    if (yr != 0)
       printf("There are %d days in month %d in year %d\n",
               daysInMonth(mth, yr), mth, yr);
  } while (yr != 0);

  return 0;
}

int daysInMonth(int month, int year){
  int numberOfDays;
  switch(month){
    case 1: case 3: case 5: case 7: case 8: case 10: case 12: 
      numberOfDays = 31; break;
    case 4: case 6: case 9: case 11: 
      numberOfDays = 30; break;
    case 2:
      if (isLeapYear(year)) numberOfDays = 29; else numberOfDays = 28;
      break;
    default: exit(-1); 
  }
  return numberOfDays;
}   

int isLeapYear(int year){
  int res;
  if (year % 400 == 0)
    res = 1;
  else if (year % 100 == 0)
    res = 0;
  else if (year % 4 == 0)
    res = 1;
  else res = 0;
  return res;
}