#include <cstdio>
#include <cmath>

struct point{
    double x;
    double y;
};

double CrossProduct(const point &p1, const point &p2){
    return p1.x * p2.y - p1.y * p2.x;
}
double PolygonArea(const point vertices[], int N){
    double sum = 0.0;

    for(int i = 1; i <= N; i++){
        sum += CrossProduct(vertices[i], vertices[(i == N) ? 1 : i + 1]);
    }
    return fabs(sum)/2.0;
}

int N;
point vertices[100001];
double Area;

int main(){

freopen("aria.in", "r", stdin);
freopen("aria.out", "w", stdout);

scanf("%d", &N);

for(int i = N; i >= 1; i--){
    scanf("%lf", &vertices[i].x);
    scanf("%lf", &vertices[i].y);
}

Area = PolygonArea(vertices, N);

printf("%.5lf", Area);

return 0;
}