trapezodial

#include<bits/stdc++.h>
using namespace std;

double f(double x)
{
    return (1/(1+(x*x)));
}

int main()
{
    double a,b,n,sum=0,h,ans,x,i;
    cin>>a>>b>>n;
    h=((b-a)/n);
    for(i=1;i<=n-1;i++)
    {
        x=a+h*i;
        sum=sum+2*f(x);
    }
    sum=sum+f(a)+f(b);
    ans=sum*(h/2);
    cout<<ans<<endl;
    return 0;
}