var L,i,j,n:1..10000;

M:1..100;

a,b:array[1..100] of integer;

s:array[1..10000] of integer;

begin

readln(L,M);

for i:=0 to L do

s[i]:=1;

for i:=1 to M do

begin

readln(a[i],b[i]);

for j:=a[i] to b[i] do

s[j]:=0;

end;

n:=0;

for i:=0 to L do

n:=n+s[i];

writeln(n);

end.