#include <fstream>
using namespace std;

ifstream fin("aria.in");
ofstream fout("aria.out");

struct point{
    int x, y;
};

int main(){
    unsigned int n, i;
    point triP1, triP2, triP3;
    int x1, x2, y1, y2, sum = 0;
    fin >> n >> triP1.x >> triP1.y >> triP2.x >> triP2.y;
    for(i = 2; i < n; ++i){
        fin >> triP3.x >> triP3.y;
        x1 = triP2.x - triP1.x;
        x2 = triP3.x - triP2.x;
        y1 = triP2.y - triP1.y;
        y2 = triP3.y - triP2.y;
        sum += x1 * y2 - x2 * y1;
        triP2.x = triP3.x;
        triP2.y = triP3.y;
    }
    fout << abs(sum / 2) + (float)(sum % 2 * 0.5);
    return 0;
}