#include<bits/stdc++.h>

#define N 105

using namespace std;

double x[N],y[N],ans;

int n;

int main()

{

    scanf("%d",&n);

    for(int i=;i<=n;i++) scanf("%lf%lf",&x[i],&y[i]);

    x[]=x[n],y[]=y[n];

    for(int i=;i<n;i++){

        ans+=0.5*(x[i]*y[i+]-y[i]*x[i+]);

    }

    printf("%d\n",int(ans));

    return ;

}

#include<iostream>

#include<cstdio>

#include<cstring>

#include<cmath>

using namespace std;

double lastx,lasty,x,y,b,S,stx,sty;

int n;

int main()

{

    scanf("%d%lf",&n,&b);

    scanf("%lf%lf",&stx,&sty);

    lastx=stx,lasty=sty;

    for(int i=;i<=n;i++){

        scanf("%lf%lf",&x,&y);

        S+=x*lasty-y*lastx;

        lastx=x,lasty=y;

    }

    S+=lasty*stx-lastx*sty;

    printf("%.4lf\n",fabs(S*b/2.0));

    return ;

}