/// What's wrong?
var a,b,c,i,s:integer;
m:array[1..1000000] of integer;
begin
s:=0;
read(a);
read(b);
for i:=1 to b do
begin
read(m[i]);
if(m[i]>a) then
s:=s+m[i]-a
else
begin
if s>m[i]+a then
s:=s-m[i]-a
else
s:=0;
end;
end;
if s<0 then writeln(0)
else writeln(s);
end.