{$N+}
program Exactness_Of_Projectile_Hit;
const  GMT=maxLongInt;
type   point=record
                   X:LongInt;
                   Y:LongInt;
             end;
       line=record
                   A:real;
                   B:real;
                   C:real;
            end;
var    Points:array[1..102] of point;
       Lines:array[1..102] of line;
       ans,AB,BC,AC,X,X1,Y1,X2,Y2:extended;
       N,i:LongInt;
       dop,dop1,Xv,Yv:extended;
       plus,minus:boolean;
procedure Quit;
begin
     writeln(0);
     halt;
end;
function min(a,b:extended):extended;
begin
     if a>b then
         min:=b else
         min:=a;
end;
function dist(a,b,c,d:extended):extended;
begin
     dop:=sqr(a-b);
     dop1:=sqr(c-d);
     dist:=sqrt(dop+dop1);
end;
begin
     ans:=GMT;
     readln(Xv,Yv,N);
     for i:=1 to N do
        readln(Points[i].X,Points[i].Y);
     {Checking if (Xv;Yv) is inside}
     Points[N+1]:=Points[1];
     for i:=1 to N do
     begin
          Lines[i].A:=Points[i+1].y-Points[i].y;
          Lines[i].B:=Points[i].x-Points[i+1].x;
          Lines[i].C:=Points[i].x*Points[i+1].y-Points[i].y*Points
[i+1].x;
     end;
     plus:=false;
     minus:=false;
     for i:=1 to N do
     begin
          if Lines[i].A*Xv+Lines[i].B*Yv>Lines[i].C then
              plus:=true else
          if Lines[i].A*Xv+Lines[i].B*Yv<Lines[i].C then
              minus:=true;
     end;
     if not (plus and minus) then
        Quit else
     {Counting the distance}
     begin
          for i:=1 to N do
          begin
               X1:=Points[i].x;
               X2:=Points[i+1].x;
               Y1:=Points[i].Y;
               Y2:=Points[i+1].Y;
               AB:=dist(x1,x2,y1,y2);
               BC:=dist(Xv,x2,Yv,Y2);
               AC:=dist(Xv,X1,Yv,Y1);
               X:=(sqr(AC)-sqr(BC)-sqr(AB))/(-2*AB);
               if (X>=0) and (x<=AB) then
                  ans:=min(ans,sqrt(sqr(BC)-sqr(X)));
          end;
          for i:=1 to N do
          begin
               X1:=Points[i].X;
               Y1:=Points[i].Y;
               ans:=min(ans,dist(X1,Xv,Y1,Yv));
          end;
     end;
     writeln(ans*2:0:3);
end.