1) var n,i,d,z:integer; s:real; begin readln(n); s:=1; d:=1; z:=1; i:=1; repeat i:=i+1; d:=d*2; z:=-z; s:=s+z/d; until i>=n; writeln('s = ',s); end.
2) var n,i,d,z:integer; s:real; begin readln(n); s:=1; d:=1; z:=1; i:=1; while i<n do begin i:=i+1; d:=d*2; z:=-z; s:=s+z/d; end; writeln('s = ',s); end.
3) var n,i,d,z:integer; s:real; begin readln(n); s:=1; d:=1; z:=1; for i:=2 to n do begin d:=d*2; z:=-z; s:=s+z/d; end; writeln('s = ',s); end.