#include <cstdio>

int n,niz[111];

int sol;

int main(void){
    scanf("%d", &n);
    for (int i=0;i<n;++i)
        scanf("%d", niz+i);
    for (int i=n-2;i+1;--i){
        if (niz[i]>=niz[i+1]){
            sol+=niz[i]-niz[i+1]+1;
            niz[i]=niz[i+1]-1;
        }
    }
    printf("%d\n", sol);
    return 0;
}