01  #include <stdio.h>
02  #include <math.h>
03  
04  int main()
05  {
06   long octalNumber, decimalNumber=0;
07   int remainder,i=1;
08  
09   printf("Enter an Octal Number : ");
10   scanf("%ld", &octalNumber);
11  
12   while(octalNumber!=0)
13   {
14    remainder = octalNumber%10;
15    decimalNumber += remainder*i;
16    octalNumber /= 10;
17    i=i*8;
18   }
19  
20   printf("Decimal Number : %ld", decimalNumber);
21   return 0;
22  }

 OUTPUT :
 
 Enter any Octal number : 256
 Decimal Number : 174