var n,m,min,b,a,max,i,j,z:integer;
    r,c: array [1..1000000] of integer;
    r2,c2: array [1..1000000] of integer;
begin
min:=100000;
max:=0;
b:=0;
a:=0;
    readln (n,m);
    for i:=1 to n do
    begin
    read(r[i]);
    read(c[i]);
    end;
    for j:=1 to n do
    begin
    for i:=1 to n do
    if r[i]<min then
    if r[i]>0 then
    begin
    min:=r[i];
    z:=i;
    end;
    r2[j]:=r[z];
    c2[j]:=c[z];
    r[z]:=0;
    min:=100000;
    end;
    for i:=1 to n do
    if c2[i]>max then max:=c2[i];
    for j:=2 to n do
    if c2[j]>=c2[j-1] then b:=b+1
    else a:=a+1;
    if a=(n-1) then writeln(max) else
    if b=(n-1) then writeln(max) else
    writeln(max+(c2[n]-c2[n-1]));
end.