import core.bitop, std.algorithm, std.ascii, std.bigint, std.conv, std.math,
    std.functional, std.numeric, std.range, std.stdio, std.string, std.random,
    std.typecons, std.container, std.format;
static import std.ascii;

// dfmt off
T lread(T = long)(){return readln.chomp.to!T();}
T[] lreads(T = long)(long n){return generate(()=>readln.chomp.to!T()).take(n).array();}
T[] aryread(T = long)(){return readln.split.to!(T[])();}
void scan(TList...)(ref TList Args){auto line = readln.split();
foreach (i, T; TList){T val = line[i].to!(T);Args[i] = val;}}
alias sread = () => readln.chomp();enum MOD = 10 ^^ 9 + 7;
alias PQueue(T, alias less = "a<b") = BinaryHeap!(Array!T, less);
// dfmt on

void main()
{
    long N = lread();
    auto A = aryread();
    foreach (i; 0 .. N)
    {
        A[i] -= i + 1;
    }
    long s = A.sum();
    long eval(long x)
    {
        return A.map!(a => abs(a - x)).sum();
    }

    long ans = long.max;

    long ok = -(10 ^^ 9);
    long ng = +(10 ^^ 9);

    while (1 < ng - ok)
    {
        long m = (ok + ng) / 2;
        if (eval(m + 1) <= eval(m))
        {
            ok = m;
        }
        else
        {
            ng = m;
        }
    }

    writeln(min(eval(ok), eval(ok - 1), eval(ok + 1)));
}
