program pachete;
type stiva=array[0..10000]of integer;
var v,ti,td,u,d:stiva;
    n,tmin,j,k,i,x:integer;
    f,g:text;
begin
assign(f,'packet.in');reset(f);
assign(g,'packet.out');rewrite(g);
readln (f,n);
for i:=1 to n do readln(f,ti[i],td[i]);
v[1]:=0;
for i:=2 to n do v[i]:=v[i-1]+ti[i-1];
d[n]:=v[n];
for i:=n-1 downto 1 do begin
                       x:=d[i+1]-td[i+1];
                       if x<=v[i] then d[i]:=x
                       else d[i]:=v[i];
                       end;
writeln(g,td[1]-d[1]);
close(f);close(g);
end.