var a: array[0..500000] of longint; Sum: array[0..500000] of longint; n: longint; procedure nhap; var i: longint; begin //assign(input,'count.inp'); reset(input); readln(n); for i := 1 to n do Read(a[i]); //close(input); end; procedure xuli; var i: longint; ans: longint; cnt: array[1..500000] of longint; begin Sum[0] := 0; for i := 1 to n do sum[i] := sum[i - 1] + a[i]; //if sum[n] div 3 =0 then write(0) //else // begin ans := 0; fillchar(cnt, sizeof(cnt), 0); for i := n - 1 downto 1 do if sum[n] - sum[i] = sum[n] div 3 then cnt[i] := cnt[i + 1] + 1 else cnt[i] := cnt[i + 1]; for i := 1 to n do if sum[i] = (sum[n] div 3) then ans := ans + cnt[i + 1]; Write(ans); // end; end; begin nhap; //assign(output,'count.out'); // rewrite(output); xuli; //close(output); end.