Exercise 2.3

#include<stdio.h>
#include<string.h>
#include<math.h>
long long htoi(char s[])
{
     int len=strlen(s);
     int i,j=0;
    long long hed,sum=0;
    for(i=len-1;i>=0;i--)
    {
        if(s[i]>='0' && s[i]<='9')
        {
            hed=s[i]-'0';
        }
        else if(s[i]>='A'  &&  s[i]<='F')
        {
            hed=(s[i]-'A')+10;
        }
        else if(s[i]>='a'  && s[i]<='f')
        {
            hed=(s[i]-'a')+10;
        }
        sum+=pow(16,j)*hed;  //otheres ot decimat always power method
        j++;
    }
    return sum;
}
int main()
{
    int i;
     char s[1000];
     scanf("%s",s);
     long long  gross=htoi(s);
     printf("%lld\n",gross);

     return 0;

}