#include <iostream>

using namespace std;

int main(){
    int a;
    int b[102];
    int br=0;
    bool o=true;
    
    cin >> a;
    
    for(int i=0;i<a;i++)
            cin >> b[i];
            
    while(o==true){
    o=false;
    for(int i=1;i<a;i++){
                    if(b[i]<=b[i-1]){
                     br+=b[i-1]-b[i]+1;
                     b[i-1]-=(b[i-1]-b[i])+1;
                     o==true;
                     }
            
    }
}
    for(int i=1;i<a;i++){
                    if(b[i]<=b[i-1]){
                     br+=b[i-1]-b[i]+1;
                     b[i-1]-=(b[i-1]-b[i])+1;
                     }
               
    }
    

    
    cout << br;
    return 0;
    }