This submission is migrated from previous version of oj.uz, which used different machine for grading. This submission may have different result if resubmitted.
#include <bits/stdc++.h>
#define fr(i, n, m) for(int i = (n); i < (m); i ++)
#define st first
#define nd second
#define pq priority_queue
#define all(x) begin(x), end(x)
using namespace std;
typedef long long ll;
typedef pair<int,int> pii;
const int mxn = 2e5 + 5;
const ll mod = 1e9 + 7;
int a[mxn];
int n;
ll C(int N, int K){
if(N == 0) return 1;
return C(N-1, K+1) + C(N-1, K)*K;
}
int main(){
cin >> n;
fr(i, 0, n){
cin >> a[i];
}
int nxt = 2;
ll ans = 1;
fr(i, 0, n){
int rem = n-i;
ans += (a[i]-1) * C(rem-1, nxt-1);
if(a[i] == nxt){
nxt++;
}
}
cout<<ans<<endl;
}
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