#include<bits/stdc++.h>
using namespace std;
#define E 1e-18
#define ll long long
#define int long long
#define db double
#define For(i,j,k) for(int i=(int)(j);i<=(int)(k);i++)
#define Rep(i,j,k) for(int i=(int)(j);i>=(int)(k);i--)
inline ll read(){
	ll x=0;char ch=getchar();bool f=0;
	for(;!isdigit(ch);ch=getchar()) if(ch=='-') f=1;
	for(;isdigit(ch);ch=getchar()) x=x*10+ch-'0';
	return f?-x:x;
}
void write(ll x){
	if(x<0) putchar('-'),x=-x;
	if(x>=10) write(x/10);putchar(x%10+'0');
}
void writeln(ll x){write(x);puts("");}
void writep(ll x){write(x);putchar(' ');}

int const N=100+3;
struct point{
	db x,y;
    point operator - (point A) const{return (point){x-A.x,y-A.y};}
    point operator + (point A) const{return (point){x+A.x,y+A.y};}
    void print(){printf("%.10lf %.10lf\n",x,y);}
}A[N];

point operator *(db x,point A){return (point){x*A.x,x*A.y};}
db CJ(point A,point B){return A.x*B.y-B.x*A.y;}//叉积 
db DJ(point A,point B){return A.x*B.x+A.y*B.y;}//点积 
db dis(point A){return sqrt(A.x*A.x+A.y*A.y);}//向量长度 
bool cmp(point A,point B){return (CJ(A,B)>0 || CJ(A,B)==0 && dis(A)<dis(B));}//按极角排序 
point CGL_1_A(point A,point B,point C){//点在直线上的投影点
	db d=dis(B-A);
	return (A+(DJ(C-A,B-A)/d/d*(B-A)));
}
point CGL_1_B(point A,point B,point C){//点关于直线的对称点
	point P=CGL_1_A(A,B,C);
	return (C+2*(P-C));
}
int CGL_1_C(point A,point B,point C){//向量的位置关系(向量AC和向量AB) 
    if (CJ(B-A,C-A)>0) return 1;//逆时针 
    if (CJ(B-A,C-A)<0) return 2;//顺时针 
    if (DJ(C-A,B-A)<0) return 3;//方向相反
	if (dis(C-A)>dis(B-A)) return 4;//方向相同 AC>AB
	if (dis(C-A)<=dis(B-A)) return 5;//方向相同 AB>AC
}
int CGL_2_A(point A,point B,point C,point D){//判断直线平行、垂直
	if (CJ(B-A,D-C)==0) return 2;//平行 
	if (DJ(B-A,D-C)==0) return 1;//垂直 
	return 0;//相交 
}
int CGL_2_B(point A,point B,point C,point D){//判断线段是否有交
	if (min(A.x,B.x)>max(C.x,D.x) || min(C.x,D.x)>max(A.x,B.x) || min(A.y,B.y)>max(C.y,D.y) || min(C.y,D.y)>max(A.y,B.y)) return 0;//快速排斥实验(两个矩形是否相交)
	return CJ(B-A,C-A)*CJ(B-A,D-A)<=0 && CJ(D-C,A-C)*CJ(D-C,B-C)<=0;//跨立实验
}
point CGL_2_C(point A,point B,point C,point D){//求直线交点
	point p1=CGL_1_A(A,B,C),p2=CGL_1_A(A,B,D);//求C、D在AB上的投影点 
	db d1=dis(C-p1),d2=dis(D-p2);//三角形的相似比 
	if (CJ(B-A,C-A)*CJ(B-A,D-A)<0) return (C+(d1/(d1+d2)*(D-C)));
	else if (d1<d2) return (D+d2/(d2-d1+E)*(C-D));
	else return (C+d1/(d1-d2+E)*(D-C));
}
db Distance(point A,point B,point C){//点到线段的距离 
	point P=CGL_1_A(A,B,C);//C在AB上的投影点 
	if (P.x>=min(A.x,B.x) && P.x<=max(A.x,B.x) && P.y>=min(A.y,B.y) && P.y<=max(A.y,B.y)) return dis(P-C);//投影点在线段上 
	return min(dis(A-C),dis(B-C));//两个端点到C的最小值 
}
db CGL_2_D(point A,point B,point C,point D){//线段到线段的距离
	point P=CGL_2_C(A,B,C,D);//求直线交点 
	if (P.x>=min(A.x,B.x) && P.x<=max(A.x,B.x) && P.y>=min(A.y,B.y) && P.y<=max(A.y,B.y) && P.x>=min(C.x,D.x) && P.x<=max(C.x,D.x) && P.y>=min(C.y,D.y) && P.y<=max(C.y,D.y)) return 0;//线段有交点 
	return min(Distance(A,B,C),min(Distance(A,B,D),min(Distance(C,D,A),Distance(C,D,B))));
}
db CGL_3_A(){//多边形的有向面积
    int n=read(),ans=0;
    for (int i=1;i<=n;i++) scanf("%lf%lf",&A[i].x,&A[i].y);
    A[n+1]=A[1];
    for (int i=1;i<=n;i++) ans+=CJ(A[i],A[i+1]);//叉积求平行四边形面积 
    printf("%.1lf\n",ans/2.0);//总面积要 ÷2
}
signed main(){
	CGL_3_A();
	return 0;
}


Measured memory_bytes: 3252