program mst;
var
        x,y,c,f:array [1..600000] of longint;
        n,m,i,j,sum,cost,v1,v2,min,max,mm,xx:longint;
procedure qsort(l,r : longint);
var     i,j,xx,tmp  : longint;
begin
        i := l; j := r; xx := c[(l+r)div 2];
        repeat
          while c[i] < xx do inc(i);
          while c[j] > xx do dec(j);
          if i <= j then
          begin
            tmp := x[i]; x[i] := x[j]; x[j] := tmp;
            tmp := y[i]; y[i] := y[j]; y[j] := tmp;
            tmp := c[i]; c[i] := c[j]; c[j] := tmp;
            inc(i); dec(j);
          end
        until i > j;
        if l < j then qsort(l,j);
        if i < r then qsort(i,r);
end;
function find(i:longint):longint;
begin
        if f[i]<>i then find:=find(f[i])
                   else find:=i;
        f[i]:=find
end;
begin
        while true do
        begin
        read(n,m);
        if (n=0)and(m=0) then halt;
        if m=0 then
         begin
          writeln(-1);
         end
        else
        if n>m+1 then
         begin
          for i:=1 to m do
           read(x[i],y[i],c[i]);
          writeln(-1);
         end
        else
        begin
        for i:=1 to m do
         read(x[i],y[i],c[i]);
        qsort(1,m);
        mm:=maxlongint;
        xx:=0;
        for i:=1 to m-n+2 do
         begin
          for j:=1 to n-1 do
           f[j]:=j;
          min:=maxlongint;
          max:=-maxlongint;
          sum:=0;
          for j:=i to m do
           begin
            v1:=find(x[j]);
            v2:=find(y[j]);
            if v1<>v2 then
             begin
              inc(sum);
              if c[j]<min then min:=c[j];
              if c[j]>max then max:=c[j];
              f[v1]:=v2;
              if sum=n-1 then break;
             end
           end;
          if (max=-maxlongint)and(min=maxlongint) then
           if i=1 then writeln(-1) else
           else if mm>max-min then mm:=max-min;
         end;
        writeln(mm);
        end;
        end;
end.