Simpson 3/8 Rule

#include<bits/stdc++.h>
using namespace std;
double f(double k){
  return 1/(1+k);
}
int main(){
  int n,i;
  double a,b,h,x,sum=0,integral=0;
  printf("\nEnter the no. of sub-intervals(MULTIPLE OF 3): ");
  scanf("%d",&n);
  printf("\nEnter the initial limit: ");
  scanf("%lf",&a);
  printf("\nEnter the final limit: ");
  scanf("%lf",&b);
  h=(b-a)/n;
  //cout<<h<<f(a)<<f(b);
  sum=f(a)+f(b);
  for(i=1;i<n;i++){
    x=a+i*h;
    if(i%3==0){
      sum=sum+2*f(x);
    }
    else{
      sum=sum+3*f(x);
    }
  }
  integral=sum*((3*h)/8);
  printf("\nThe integral is: %lf\n",integral);
}