abc102_c_linear_approximation.cpp

// Linear Approximation | https://atcoder.jp/contests/abc102/tasks/arc100_a
// Time: O(n log n); extra space: O(n).
#include <algorithm>
#include <cstdlib>
#include <iostream>
#include <vector>

void solve() {
    int n;
    std::cin>>n;
    std::vector<long long>a(n);
    for(int i=0;i<n;++i) {
        std::cin>>a[i];
        a[i]-=i+1;
    }
    std::sort(a.begin(),a.end());
    long long ans=0;
    for(long long x:a)ans+=std::abs(x-a[n/2]);
    std::cout<<ans<<'\n';
}
int main() {
    std::ios::sync_with_stdio(false);
    std::cin.tie(nullptr);
    solve();
}