C code to Convert Hexadecimal String to Decimal number

//This program takes a hexadecimal string and converts it into the decimal string//

#include<stdio.h>
#include<string.h>
#include<conio.h>

int main()
{
char *str;
clrscr();
printf("\nEnter a hexadecimal string\n");
gets(str);

printf("\nthe corresponding decimal value is %d",htoi(str));
getch();
return(0);
}

int power (int a, int b) // utility function
{
int res = 1 ;
while( b>0 )
{
res *= a ;
b--;
}
return ( res );
}

int htoi ( char* s )
{
int result = 0,new_result;
//int value =0;
int len = strlen(s);
int exp = len;
char c;
int i;

  for( i=0; i<len; i++)
   {

    c = s[i];

     if ( c >='1' && c<='9')
new_result = (int)( (c - '0') * power(16, exp-1) );

     else
if (c >='A' && c<='F')
  new_result = (int)( (c - 55) * power(16, exp-1) );

    exp --;
    result += new_result;

   }

return (result);
}