Calculate the Area

//Bismillahir Rahmanir Rahim
/******Harun Or Rashid******/
/***********Template Starts Here***********/
#include<iostream>
#include<cstdio>
#include<cstring>
#include<cmath>
#include<cctype>
#include<cstdlib>
#include<string>
#include<vector>
#include<set>
#include<stack>
#include<queue>
#include<map>
#include<algorithm>
using namespace std;

#define MEM(a,b) memset(a,b,sizeof(a))
#define CLR(a) memset(a,0,sizeof(a))
#define MAX(a,b) ((a)>(b)?(a):(b))
#define MIN(a,b) ((a)>(b)?(b):(a))
#define ABS(a) ((a)>0? (a):(-(a)))
#define SQ(a) ((a)*(a))
#define SZ(a) (int)a.size()
#define FORN(i,n) for(i=1;i<=n;i++)
#define FORAB(i,a,b) for(i=a;i<=b;i++)
#define ALL(V) V.begin(),V.end()
#define pb(a) push_back(a)
#define pr(a) cout<<a<<endl
#define pf printf
#define sf scanf
#define swap(a,b) (a ^= b, b ^= a, a ^= b)
#define PI acos(-1)
#define x first
#define y second
#define eps 1e-7

typedef int D;
typedef long long int LLD;
typedef unsigned long long int LLU;
typedef vector<D> VI;
typedef set<D> SI;
typedef vector<D>::iterator Viti;

/***********Template Ends Here***********/

int main()
{
    int n,i,x1,y1,x2,y2,result;
    double area=0,cross;
    pair<int,int>point;
    vector<pair<int,int> >V;
    cin>>n;
    for(i=0; i<n; i++)
    {
        cin>>point.x>>point.y;
        V.pb(point);
    }
    for(i=1; i<n-1; i++)
    {
        x1=V[i].x-V[0].x;
        y1=V[i].y-V[0].y;
        x2=V[i+1].x-V[0].x;
        y2=V[i+1].y-V[0].y;
        cross=x1*y2-x2*y1;
        area+=cross;
    }
    result=int(ABS(area/2));
    cout<<result<<endl;
    return 0;
}