{
ZADATAK: meteor
JEZIK: PASCAL
}

var x,y,r,d1,d2,d:double;
        p,x1,x2,y1,y2,i,j:longint;
begin
        readln(x,y,r);
        x1:=trunc(x-r);
        x2:=trunc(x+r+0.999);
        y1:=trunc(y-r);
        y2:=trunc(y+r+0.999);
        if (x=trunc(x)) and (y=trunc(y))
        then begin
        d1:=0.0;
        p:=0;
        for i:=y1+1 to (y1+y2)div 2 do
        begin
             d2:=sqrt(r*r-(i-y)*(i-y));
             if d1>d2 then d:=d1 else d:=d2;
             j:=x1+1;
             while j<=x-d do
             begin
                inc(j);
                p:=p-4;
             end;
             d1:=d2;
        end;
        p:=p+(x2-x1)*(y2-y1);
        end
        else if (y=trunc(y))
        then begin
        d1:=0.0;
        p:=0;
        for i:=y1+1 to (y1+y2)div 2 do
        begin
             d2:=sqrt(r*r-(i-y)*(i-y));
             if d1>d2 then d:=d1 else d:=d2;
             j:=x1+1;
             while j<=x-d do
             begin
                inc(j);
                p:=p-2;
             end;
             j:=x2-1;
             while j>=x+d do
             begin
                dec(j);
                p:=p-2;
             end;
             d1:=d2;
        end;
        p:=p+(x2-x1)*(y2-y1);
        end
        else if (x=trunc(x))
        then begin
        d1:=0.0;
        p:=0;
        for i:=y1+1 to y2-1 do
        begin
             d2:=sqrt(r*r-(i-y)*(i-y));
             if d1>d2 then d:=d1 else d:=d2;
             j:=x1+1;
             while j<=x-d do
             begin
                inc(j);
                p:=p-2;
             end;
             j:=x2-1;
             d1:=d2;
        end;
             j:=x1+1;
             while j<=x-d2 do
             begin
                inc(j);
                p:=p-2;
             end;
        p:=p+(x2-x1)*(y2-y1);
        end
        else begin
        d1:=0.0;
        p:=0;
        for i:=y1+1 to y2-1 do
        begin
             d2:=sqrt(r*r-(i-y)*(i-y));
             if d1>d2 then d:=d1 else d:=d2;
             j:=x1+1;
             while j<=x-d do
             begin
                inc(j);
                dec(p);
             end;
             j:=x2-1;
             while j>=x+d do
             begin
                dec(j);
                dec(p);
             end;
             d1:=d2;
        end;
             j:=x1+1;
             while j<=x-d2 do
             begin
                inc(j);
                dec(p);
             end;
             j:=x2-1;
             while j>=x+d2 do
             begin
                dec(j);
                dec(p);
             end;
        p:=p+(x2-x1)*(y2-y1);
        end;
        writeln(p);
end.
