#include <stdio.h>
#include <math.h>

struct Point{
    long double x;
    long double y;
}vertices[100000];

long double CrossProduct(struct Point a, struct Point b){
    return a.x*b.y - a.y*b.x;
}
long double PolygonArea(struct Point vertices[], int N){
    long double sum = 0.0;
    int i;

    for(i=0; i<N; i++){
        sum += CrossProduct(vertices[i], vertices[(i+1)%N]);
    }
    return fabs(sum)/2.0;
}

int main(){

FILE *file1, *file2;
int N, i;

file1 = fopen("aria.in", "r");
file2 = fopen("aria.out", "w");

fscanf(file1, "%d", &N);

for(i=0; i<N; i++){
    fscanf(file1, "%llf", &vertices[i].x);
    fscanf(file1, "%llf", &vertices[i].y);
}
fprintf(file2, "%.5llf", PolygonArea(vertices, N));

return 0;
}