program b16dayfiboci; var n,i,j,d,max:longint; a:array[1..10000] of longint; begin readln(n); max:=0; for i:=1 to n do read(a[i]); for i:=3 to n do begin if ((a[i-2])+(a[i-1]))=a[i] then begin j:=i; d:=2; while (a[j-2]+a[j-1])=a[j] do begin inc(d); inc(j); end; if d>max then max:=d; end; end; if max=0 then write(-1) else write(max); end.