int click  = 0;
int x_1 = 0;
int x_2 = 0;
int y_1 = 0;
int y_2 = 0;
void setup()
{
  
size(500,500);  
}

void draw() {

}

void mouseClicked() {
if (click == 0){
x_1 = mouseX;
y_1 = mouseY;
}
if (click == 1){
x_2 = mouseX;
y_2 = mouseY;
}
click = click + 1;
if (click == 2){
line(x_1,y_1,x_2,y_2);  
click = 0;
}
}