var
a:array[1..100]of real;
n,m,i:longint;
d:real;
begin
readln(n,m);
readln(d,a[1],a[n]);
for i:=1 to n-2 do a[i+2]:=a[i]-2*a[i+1]+2*d;
writeln(a[m]:0:3);
end.

5个点是如何过的
剩余5个点是怎么回事