uses crt;
var a:array[52..101]of integer;
i,n,t:integer;
begin
clrscr;
repeat
write('Nhap n='); readln(n);
until (51<n) and (n<102);
for i:=52 to n do
begin
write('A[',i,']='); readln(a[i]);
end;
t:=0;
for i:=52 to n do
if i mod 2=0 then t:=t+a[i];
writeln(t);
readln;
end.