Program sate;
var fi,fo:text;
    s,a,d,b:array[1..100025] of integer;
    n,m,x,y,i : integer;
begin
assign(fi,'sate.in'); reset(fi);
assign(fo,'sate.out'); rewrite(fo);
 readln(fi,n); readln(fi,m);
 readln(fi,x); readln(fi,y);
   s[x]:=1;
   for i:=1 to m do begin
   readln(fi,a[i]); readln(fi,b[i]); readln(fi,d[i]);  end;
    while (s[y]=0) do begin
                        for i:=1 to m do
                        if (s[a[i]]<>0) and (s[b[i]]=0) then begin
                                                    s[b[i]]:=s[a[i]]+d[i];
       end else
        if (s[a[i]]=0) and (s[b[i]]<>0) then
                                    s[a[i]]:=s[b[i]]-d[i];
              end;
    writeln(fo,s[y]-1);
 close(fi); close(fo);
 readln;
 end.