MECUNG1223 pascal

var n,m:int64;
    i:longint;
    f:array[1..70] of int64;
procedure giai;
begin
    f[1]:=1;f[2]:=2;f[3]:=4;
    for i:=4 to n do
        f[i]:=f[i-1]+f[i-2]+f[i-3];
    if m>f[n] then
        writeln(m-f[n])
    else
        writeln(0);
end;
begin
    assign(input,'mecung1223.inp');reset(input);
    assign(output,'mecung1223.out');rewrite(output);
    readln(m);
    readln(n);
    giai;
end.