Revise paste Convex Polygon

#include<stdio.h>

typedef struct points2D{

double x;
    double y;
}pts;

double tri_area(pts p1,pts p2,pts p3)
{
    double A;

//Calculating the area of triangles
    A=0.5*((p1.x*p2.y)+(p2.x*p3.y)+(p3.x*p1.y)-(p1.y*p2.x)-(p2.y*p3.x)-(p3.y*p1.x));

return A;
}

int main()
{
    int N,i,flag=0;
    scanf("%d",&N);
    pts p[N],p1,p2,p3;
    double arr[N];
    for(i=0;i<N;i++)
    {
        scanf("%lf %lf",&p[i].x,&p[i].y);
    }
    for(i=0;i<N-2;i++)
    {
//Taking 3 points and passing them to the tri_area() function and storing the area into the arr[]
        p1.x=p[i].x;
        p1.y=p[i].y;
        p2.x=p[i+1].x;
        p2.y=p[i+1].y;
        p3.x=p[i+2].x;
        p3.y=p[i+2].y;
        arr[i]=tri_area(p1,p2,p3);
    }
//Set the condition to find out whether all the area of the triangles are -ve 
    if(arr[0]<0)
    {
        for(i=1;i<N-2;i++)
        {
            if(arr[i]>=0)
            {
                flag=1;
                break;
            }
        }
    }
//Set the condition to find out whether all the area of the triangles are +ve
    else
    {
        for(i=1;i<N-2;i++)
        {
            if(arr[i]<0)
            {
                flag=1;
                break;
            }
        }
    }
    if(flag==0)
        printf("Convex Polygon exists\n");
    else if(flag==1)
        printf("No convex polygon possible\n");
    return 0;
}