var minutes, carsAtMin, stay, i: byte;
    countOfCars: array[0..100] of integer;
BEGIN
readln(carsAtMin, minutes);
stay := 0;
for i:=1 to minutes do
    readln(countOfCars[i]);
for i:=1 to minutes do
    begin
    countOfCars[i] := countOfCars[i] + stay;
    stay := 0;
    if (countOfCars[i] - carsAtMin > 0) then stay := countOfCars[i] - carsAtmin;
    end;
if (stay > 0) then write(stay)
else write('0');
END.