c program to convert hexadecimal value to decimal value

#include<stdio.h>

#include<stdlib.h>

void x(char *);
int f1(int,int);
int hex()
{
    char a[10];
    char* b;
    printf("\nEnter the hexdecimal value : ");
    scanf("%s",&a);
    x(a);
return 0;
}
void x(char *a)
{
int sum=0,b;
int c=strlen(a);
b=c;
for (int i=0;i<c;i++)
{
if(a[i] >= '0' && a[i] <= '9')
{
    sum = sum + f1(a[i]-'0',b);
    b--;
}
else if(a[i] >= 'a' && a[i] <= 'f')
{   
    sum = sum + f1(a[i]-'a'+10,b);
    b--;
}
else if(a[i] >= 'A' && a[i] <= 'F')
{   
    sum = sum + f1(a[i]-'A'+10,b);
    b--;
}
else printf("Error value entered\n");
}
printf("\nResultant decimal value is :   %d  \n",sum);
}

int f1(int a, int b)
{
int d;
int p=1;
if(b != 0){
for (int i=1;i<b;i++)
    p = p*16;
}
    d=a*p;
return d;
}