#include <stdio.h>
#include <bits/stdc++.h>
#include <atcoder/all>
using namespace atcoder;
using mint = modint998244353;
using namespace std;
#define rep(i,n) for (int i = 0; i < (n); ++i)
#define Inf 1000000000000000001
	
long long get(long long a,long long b,long long c,long long d){
	return abs(a*d-b*c);
}

int main(){
	
	int N;
	cin>>N;
	
	vector<long long> X(N),Y(N);
	rep(i,N){
		cin>>X[i]>>Y[i];
	}
	
	long long ans = 0LL;
	
	rep(i,N){
		ans += get(X[i]-X[0],Y[i]-Y[0],X[(i+1)%N]-X[0],Y[(i+1)%N]-Y[0]);
	}
	
	cout<<ans<<endl;
	
    return 0;
}