var i,j,l,m,x,y,t:longint;
a:array[0..10001]of longint;
begin
readln(l,m);
for i:=1 to m do begin
readln(x,y);
for j:=x to y do a[j]:=1;
end;
for i:=1 to l do if a[i]=0 then t:=t+1;
writeln(t);
end.