Simpson's 1/3rd Rule to find Integration of a Function

#include<iostream>
#include<math.h>
using namespace std;
float f(float x)
{
    return 1/(1+x*x);

}
int main()
{
    int i,n;
    float x0,xn,h,sum;
    cout<<"Enter no of Intervals: ";

    cin>>n;
    cout<<"Enter x0 & xn: ";
    cin>>x0>>xn;
    h=(xn-x0)/n;
    sum=f(x0)+f(xn)+4*f(x0+h);
    for(i=3;i<=n-1;i+=2)
        sum+=4*f(x0+i*h)+2*f(x0+(i-1)*h);
    cout<<"Integral Value="<<(h/3)*sum;
    return 0;
}