var j,z,i,x,y,n,numar:longint;
f,g:text;
begin
assign(f,'operatii.in');reset(f);
assign(g,'operatii.out');rewrite(g);
readln(f,n);
numar:=0;
x:=0;
for i:=1 to n do
begin
read(f,y);
z:=y-x;
if y>x then 
for j:=1 to z do numar:=numar+z;
x:=y;
end;
write(g,numar);
close(f);
close(g);
end.