Problem Relating Factorial Using Turbo-C

Equation:
            4-(2/1!)+(3/2!)-(4/3!).......(n/n-1!)
 Sample:
           n=4/*number of series*/
           Equation: 4-(2/1!)+(3/2!)+(4/3!)
           Answer: 2.83333

 Solution:

float r,sum=0,facp;
    int n,o,p;
    printf("Enter Number: ");
    scanf("%d",&n);
    for(o=2;o<=n;o++){
        facp=1;
            for(p=o-1;p>1;p--){ facp*=p; }
                                r=o/facp;
                                if(o%2==0){r=r*(-1); sum+=r;}
                                else{ sum+=r;}
            }             
  printf("\n\n\tResult: %f\n\n\n",4+sum);