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#include <bits/stdc++.h>                                                        
using namespace std;   

int main()
{
    ios_base::sync_with_stdio(0);                                               
    cin.tie(0);                                                                 
    cout.tie(0);  
    
    int n, k;
    vector<long long> fragmenty;
    long long pop_fragment = 0;
    long long odp=0;
    
    cin>>n>>k;
    fragmenty.resize(n);
    for (int i = 0; i< n; i++){
        cin>>fragmenty[i];
        if (i>0 && (fragmenty[i-1] - k > fragmenty[i])){
            odp += fragmenty[i-1] - k - fragmenty[i];
            fragmenty[i] = fragmenty[i-1] - k; 
        }
            
        
    }
    
    for (int i = n-2; i>=0; i--){
        if(fragmenty[i+1]>fragmenty[i] + k){
            odp += fragmenty[i+1] - k - fragmenty[i];
            fragmenty[i] = fragmenty[i+1] - k;
        }
            
    }
    
    cout<<odp;
    
    return 0;
}