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| #include <bits/stdc++.h>
using namespace std;
struct point{
double x;
double y;
};
int main(){
int n;
cin>>n;
point p[n];
for(int i=0;i<n;i++)cin>>p[i].x>>p[i].y;
double a = 0.0;
for(int i=0;i<n;i++){
if(i==n-1){
a+=(p[i].x)*(p[0].y)-(p[0].x)*(p[i].y);
}
else{
a+=(p[i].x)*(p[i+1].y)-(p[i+1].x)*(p[i].y);
}
}
a = abs(a/2);
cout<<fixed<<setprecision(2)<<a<<endl;
}
|