# include <iostream>

using namespace std;

int main () {
    int n,bod=0;
    cin>>n;
    int raz[n];
    for (int i=0; i<n; i++) cin>>raz[i];
    for (int i=n-1; i>=1; i--) {
        int b=0;
        if (raz[i]<=raz[i-1]) b=raz[i-1]-raz[i]+1;
        bod+=b;
        raz[i-1]-=b;
    }
    cout<<bod<<endl;

    return 0;
}