Inter

double llX(double x1,double y1,double x2,double y2,double x3,double y3,double x4,double y4){
    return ((x1*y2-y1*x2)*(x3-x4)-(x1-x2)*(x3*y4-y3*x4))/((x1-x2)*(y3-y4)-(y1-y2)*(x3-x4));
}
double llY(double x1,double y1,double x2,double y2,double x3,double y3,double x4,double y4){
    return ((x1*y2-y1*x2)*(y3-y4)-(y1-y2)*(x3*y4-y3*x4))/((x1-x2)*(y3-y4)-(y1-y2)*(x3-x4));
}