var
a:array [1..20] of integer;
i,j: integer;
s:real;
begin
writeln('vv masiv');
for i:=1 to 20 do
begin
a[i]:=random(20);
write(' ',a[i]);
end;
writeln();
s:=0;
for j:=1 to 20 do
for i:=1 to 20 do
if a[j]=a[i]-1 then s:=s+1;
s:=s/2;
writeln(s:3:3);
readln;
end.