day calculator calendar

#include<stdio.h>
main()
{

    int y,m,d,t,f,s=0,i,j,k,d1=0,n,b=0,a;
    printf("enter year:");
    scanf("%d",&y);
    printf("enter month:");
    scanf("%d",&m);
    if(m<=0 || m>12)
    {
        printf("invalid month");
        exit(0);
    }
    printf("enter day:");
    scanf("%d",&d);
    if(d<=0 || d>31)
    {
        printf("invalid date");
        exit(0);
    }
    y=y-1;
    y=y%400;
    t=y/4;
    f=y/100;
    i=t-f;
    j=y-i;
    i=2*i;
    k=i+j;
    n=k%7;
    s=s+n;
   if(m==2)
   d1=31;
   else if(m>=3)
   d1=59;
   for(a=4;a<=m;a++)
   {
       if(b==0)
       {
           d1=d1+31;
           if(a==8)
           b=0;
           else
           b=1;
       }
       else
       {
           d1=d1+30;
           b=0;
       }
   }
    d1=d1+d;
    k=d1%7;
    s=s+k;
    s=s%7;
    printf("\n");
    switch(s)
    {
        case 0:
        printf("sunday");
        break;
        case 1:
        printf("monday");
        break;
        case 2:
        printf("tuesday");
        break;
        case 3:
        printf("wednesday");
        break;
        case 4:
        printf("thursday");
        break;
        case 5:
        printf("friday");
        break;
        case 6:
        printf("saturday");
        break;
    }

}