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#include <bits/stdc++.h>

using namespace std;

typedef long long ll;
typedef vector<int> vi;

int main(){
    cin.tie(0)->sync_with_stdio(0);
    int n,k;
    cin >> n >> k;
    
    vi gr(n);
    for(int &g : gr)
        cin >> g;
    
    ll ans=0;
    for(int i=1; i<n; ++i){
        if(gr[i-1] - gr[i] > k){
            ans += gr[i-1]-gr[i]-k;
            gr[i] = gr[i-1] - k; 
        }
    }
    for(int i=n-1; i>=1; --i){
        if(gr[i] - gr[i-1] > k){
            ans += gr[i]-gr[i-1]-k;
            gr[i-1] = gr[i] - k; 
        }
    }
    
    cout << ans;
}