friend double operator ^ (Point a,Point b){return a.x*b.y-a.y*b.x;}

double TriangleArea(V l1,V l2){return fabs((l1.end-l1.start)^(l2.end-l2.start))/2;
}

//简单多边形面积，由n个点构成Dots顶点集，按顺序存储。
double Poly_Area(){double ans=0;for(int i=1;i<n;++i){ans+=(Dots[i]-Dots[0])^(Dots[i+1]-Dots[0]);}return fabs(ans)/2.0;
}

//简单多边形的面积，Dots[]为顶点集，n为多边形的顶点个数。
double Poly_Area2(){Dots[n]=Dots[0];double ans=0;for(int i=0;i<n;++i){ans+=(Dots[i])^(Dots[i+1]);}return fabs(ans)/2.0;
}

// luogu-judger-enable-o2
//#pragma comment(linker, "/STACK:1024000000,1024000000")#include<stdio.h>
#include<string.h>
#include<math.h> //#include<map>
#include<set>
#include<deque>
#include<queue>
#include<stack>
#include<bitset>
#include<string>
#include<fstream>
#include<iostream>
#include<algorithm>
using namespace std; #define ll long long
#define Pair pair<int,int>
//#define max(a,b) (a)>(b)?(a):(b)
//#define min(a,b) (a)<(b)?(a):(b)
#define clean(a,b) memset(a,b,sizeof(a))// 水印
//std::ios::sync_with_stdio(false);
//  register
const int MAXN=1e5+10;
const int INF32=0x3f3f3f3f;
const ll INF64=0x3f3f3f3f3f3f3f3f;
const int MOD=998244353;
const double PI=acos(-1.0);
const double EPS=1.0e-12;struct Point{double x,y;Point(double _x=0,double _y=0){x=_x;y=_y;}friend Point operator + (const Point &a,const Point &b){return Point(a.x+b.x,a.y+b.y);}friend Point operator - (const Point &a,const Point &b){return Point(a.x-b.x,a.y-b.y);}friend double operator ^ (Point a,Point b){return a.x*b.y-a.y*b.x;}
}Dots[MAXN];
struct V{Point start,end;double ang;V(Point _start=Point(0,0),Point _end=Point(0,0),double _ang=0.0){start=_start;end=_end;ang=_ang;}friend V operator + (const V &a,const V &b){return V(a.start+b.start,a.end+b.end);}friend V operator - (const V &a,const V &b){return V(a.start-b.start,a.end-b.end);}
}Edge[MAXN],stk[MAXN];
double Seare[MAXN];
int n;int Parellel(const V &a,const V &b){return fabs((a.end-a.start)^(b.end-b.start))<EPS;
}
Point LineInterDot(const V &l1,const V &l2){Point p;double S1=(l2.end-l1.start)^(l2.start-l1.start);double S2=(l2.start-l1.end)^(l2.start-l1.start);p.x=(l1.start.x*S2+l1.end.x*S1)/(S1+S2);p.y=(l1.start.y*S2+l1.end.y*S1)/(S1+S2);return p;
}
double TriangleArea(V l1,V l2){return fabs((l1.end-l1.start)^(l2.end-l2.start))/2;
}
double Poly_Area(){double ans=0;for(int i=1;i<n;++i){ans+=(Dots[i]-Dots[0])^(Dots[i+1]-Dots[0]);}return fabs(ans)/2.0;
}
double Poly_Area2(){Dots[n]=Dots[0];double ans=0;for(int i=0;i<n;++i){ans+=(Dots[i])^(Dots[i+1]);}return fabs(ans)/2.0;
}
int main(){int n,q;cin>>n>>q;for(int i=1;i<=n;++i){cin>>Dots[i].x>>Dots[i].y;}for(int i=1;i<n;++i){Seare[i]=Seare[i-1]+((Dots[i]-Dots[1])^(Dots[i+1]-Dots[1]))/2.0;}double sum=Seare[n-1],ans=0;
//  for(int i=1;i<n;++i){
//      cout<<Seare[i]<<" ";
//  }cout<<endl;while(q--){int a,b;double res=0;cin>>a>>b;if(a>b) swap(a,b);if(a+1==b) continue;res=Seare[b-1]-Seare[a-1];res-=((Dots[a]-Dots[1])^(Dots[b]-Dots[1]))/2;ans=max(ans,min(res,sum-res));}cout<<ans<<endl;
}
/*
4 2
0.5 0.5
10.5 0.5
10.5 10.5
0.5 10.5
1 3
4 2
*/