b1=new Array();
b2=new Array();

if (window.document.images)

for(i=1; i<11; i++) {

b1[i]=new Image();
b1[i].src='b'+i+'a.gif';
b2[i]=new Image();
b2[i].src='b'+i+'b.gif'}

function N(n){if(window.document.images)if(b2[n]){document.images['a'+n].src = b2[n].src}}

function F(n){if(window.document.images)if(b1[n]){document.images['a'+n].src=b1[n].src}}