HackerEarth Sorted Arrays problem solution

#include<bits/stdc++.h>
#define LL long long int
#define M 1000000007
#define MM 1000000009
#define reset(a) memset(a,0,sizeof(a))
#define rep(i,j,k) for(i=j;i<=k;++i)
#define per(i,j,k) for(i=j;i>=k;--i)
#define print(a,start,end) for(i=start;i<=end;++i) cout<<a[i];
#define endl "n"
#define inf 100000000000000
LL pow(LL a,LL b,LL m){LL x=1,y=a;while(b > 0){if(b%2 == 1){x=(x*y);if(x>m) x%=m;}y = (y*y);if(y>m) y%=m;b /= 2;}return x%m;}
LL gcd(LL a,LL b){if(b==0) return a; else return gcd(b,a%b);}
LL gen(LL start,LL end){LL diff = end-start;LL temp = rand()%start;return temp+diff;}
using namespace std;
int a[1000001];
int main()
  {
    ios_base::sync_with_stdio(0);
    int n;
    LL ans = 0;
    cin >> n;
    a[0] = 0;
    for(int i = 1; i <= n ; i++)
      {
        cin >> a[i];
        if(a[i] <= a[i - 1])
          {
            ans = ans + a[i - 1] - a[i] + 1;
            a[i]  = a[i - 1] + 1;
          }
      }
    cout << ans;
  }