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

const double eps = 1e-10;

int dcmp(double x) {
        if(fabs(x) < eps) return 0;
        return x < 0 ? -1 : 1;
}

bool same(double a, double b) {return dcmp(a - b) == 0;}
#define Vector P
struct P {
        double x, y;
        P(double x = 0, double y = 0): x(x), y(y) {}
        P operator + (P b) {return P(x + b.x, y + b.y);}
        P operator - (P b) {return P(x - b.x, y - b.y);}
        P operator * (double b) {return P(x * b, y * b);}
        P operator / (double b) {return P(x / b, y / b);}
        double operator * (P b) {return x * b.x + y * b.y;} // Dot
        double operator ^ (P b) {return x * b.y - y * b.x;} // Cross
        double abs() {return hypot(x, y);}
        P unit() {return *this / abs();}
        P spin(double o) {
                double c = cos(o), s = sin(o);
                return P(c * x - s * y, s * x + c * y);
        }
};

struct Line {
        //ax + by + c = 0
        double a, b, c, theta;
        P pa, pb;
        Line(): a(0), b(0), c(0), theta(0), pa(), pb() {}
        Line(P pa, P pb): a(pa.y - pb.y), b(pb.x - pa.x), c(pa ^ pb), theta(atan2(-a, b)), pa(pa), pb(pb) {}        P projection(P p) {return pa + (pb - pa).unit() * ((pb - pa) * (p - pa) / (pb - pa).abs());}                P reflection(P p) {return p + (projection(p) - p) * 2;}
        double get_ratio(P p) {return (p - pa) * (pb - pa) / ((pb - pa).abs() * (pb - pa).abs());}
        P dis(P p) {return ((pb - pa) ^ (p - pa)) / (pb - pa).abs();} // directed distance
};

struct Circle {
        P c;
        double r;
        Circle(P c, double r = 0): c(c), r(r) {}
};
bool onsegment(P p, P a, P b) {
        return dcmp((a - p) ^ (b - p)) == 0 && dcmp((a - p) * (b - p)) <= 0;
}
bool segment_intersection(P p1, P p2, P p3, P p4) { // end points are not allowed
        return dcmp((p2 - p1) ^ (p3 - p1)) * dcmp((p2 - p1) ^ (p4 - p1)) < 0
                && dcmp((p4 - p3) ^ (p1 - p3)) * dcmp((p4 - p3) ^ (p2 - p3)) < 0;

}
bool parallel(Line l1, Line l2) {return same(l1.a * l2.b, l1.b * l2.a);}
P line_intersection(Line l1, Line l2) {
        return P(-l1.b * l2.c + l1.c * l2.b, l1.a * l2.c - l1.c * l2.a) / (-l1.a * l2.b + l1.b * l2.a);     }
double Area(vector<P> &p) {
        double res = 0;
        for(int i = 1; i < (int)p.size() - 1; i++)
                res += (p[i] - p[0]) ^ (p[i + 1] - p[0]);
        return res * 0.5;
}
int main() {
        ios_base::sync_with_stdio(false); cin.tie(0);
        int n; cin >> n;
        vector<P> vec;
        for(int i = 0; i < n; i++) {
                P p; cin >> p.x >> p.y;
                vec.push_back(p);
        }
        cout << fixed << setprecision(1) << Area(vec) << '\n';
        return 0;
}
