C Solve Problem

// s = 1- 2x + 3xpow2 - 4xpow3 + .........
#include <stdio.h>
#include <math.h>
void main( )
{
   float x,s,r ;
   int a,b,n ;
   int i ;

   printf ("enter number of term after ") ;
   scanf ("%d",&n) ;
   printf ("enter value x = ") ;
   scanf ("%f",&x) ;

      s = 0 ;

   for ( i=1 ; i<=n ; i++ )

   {
       a = 1 + i ;
       b = i ;

       r = a * pow(x,b) ;

          if ( i % 2 == 0)

              s = s + r ;
         
          else

              s = s - r ;           
                                                                          //first tirm   -2x   (s = 0 - 2x)
   }
  


                s = 1 + s ;

       printf ("sum = %10.2f \n",s) ;

}