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#include <iostream>
using namespace std;

int a[1007];
int b[1007];

int main() {
    int n, k;
    cin >> n >> k;

    for(int i = 0; i < n; i++) cin >> a[i];

    for(int i = 0; i < n; i++) b[i] = a[i];

    for(int i = 1; i < n; i++){
        b[i] = max(b[i], b[i-1] - k);
    }

    for(int i = n-2; i >= 0; i--){
        b[i] = max(b[i], b[i+1] - k);
    }

    long long ans = 0;
    for(int i = 0; i < n; i++){
        ans += (b[i] - a[i]);
    }

    cout << ans << endl;
    return 0;
}