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.
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