Something

x4 := 1;
x3 := 1;
x2 := 1;
x5 := x1 - 1;
loop x5 do
  x2 := x4 + x3;
  x3 := x4;
  x4 := x2;
end
x2 := x3 + x4;

x3 := 0;
x4 := 0;
x5 := 0;