int numSteps = 20;
int stepSize = 20;

int N = (1+numSteps) * stepSize;
size(N, N);
background(255);

fill(200);  // light gray

beginShape();
// climb up the left side
for (int j=0; j < numSteps; j++) {
  float x = 0.5*(0.5+j)*stepSize;
  float y = (numSteps-j+0.5)*stepSize;
  vertex(x,y);
  vertex(x,y-stepSize);  
}
// climb down the right side
for (int j=0; j < numSteps; j++) {
  float x = 0.5*(1.5+j+numSteps)*stepSize;
  float y = (0.5+j)*stepSize;
  vertex(x,y);
  vertex(x,y+stepSize);  
}
endShape(CLOSE);