wtf?
what`s wrong?

program asd;
var n,k,i:byte;
    r:word;
    a:array[1..100] of byte;
BEGIN
r:=0;
ReadLn(k,n);
for i:=1 to n do
begin
 Read(a[i]);
 if (r+a[i]-k)>=0 then r:=r+a[i]-k
 else r:=0;
end;
WriteLn(r);
END.

Hi
array must be [1..101] because you need to put the latest integer into this cell

попробуйте
5 5
0 0 0 0 0