var
l,n,m,i,w,l,r,mid,pos,ans:longint;
a:array[1..55555]of longint;
function ok(x:longint):boolean;
begin
pos:=0;w:=0;
for i:=1 to n do
begin
if (a[i]-pos<x) then w:=w+1
else pos:=a[i];
end;
ok:=w;
end;
begin
readln(l,n,m);
for i:=1 to n do
readln(a[i]);
a[n+1]:=l;
r:=l;
for l:=1 to r do
begin
if ok(mid=l+r)>1 then ans:=mid; l:=mid+1
else r:=mid-1;
end;
writeln(ans);
end.