first = 1;     last = 22;     current = 1;                
function nextPicture() {         // Hide current picture        
 object = document.getElementById('slide' + current);         
object.style.display = 'none';                        // Show next picture, if last, loop back to front         	

	if (current == last) { 
		current = 1; }         				
		else { current++ }         			
	object = document.getElementById('slide' + current);         
	object.style.display = 'block';	setTimeout(nextPicture, 2000);      
	}      

function previousPicture() {         // Hide current picture         
	object = document.getElementById('slide' + current);         
	object.style.display = 'none';                        		
	
	if (current == first) { 
		current = last; 
		}         				
	else { current--; 
	}         						
	
	object = document.getElementById('slide' + current);         		
	object.style.display = 'block';     	
	}