calendar

#include <stdio.h>
#include <stdlib.h>
int main(void) {
  int y, m;
  setbuf(stdout, NULL);
  for (;;) {
    fputs("year? ", stdout);
    if (scanf("%d", &y) != 1) { exit(1); }
    if (1581 < y) { break; }
  }
  for (;;) {
    fputs("month? ", stdout);
    if (scanf("%d", &m) != 1) { exit(1); }
    if (0 < m && m < 13) { break; }
  }
  int w = 0;
  {
    int yy = y, mm = m;
    if (mm < 3) { --yy, mm += 12; }
    w = (yy + yy / 4 - yy / 100 + yy / 400 + (13 * mm + 8) / 5 + 1) % 7;
  }
  int days =
      (m == 4 || m == 6 || m == 9 || m == 11)
          ? 30
          : (m == 2) ? (y % 400 == 0 || (y % 4 == 0 && y % 100 != 0) ? 29 : 28)
                     : 31;
  puts(""), puts(" Sun Mon Tue Wed Thu Fri Sat");
  for (int d = 1, c = 0; d <= days;) {
    if (0 < w--) {
      fputs("    ", stdout), ++c;
      continue;
    }
    printf("%4d", d++);
    if (++c % 7 == 0) { puts(""); }
  }
}