Problem p03347 (C++)

Buggy submission (status: Runtime Error):
#include <algorithm>
#include <bitset>
#include <complex>
#include <cstring>
#include <deque>
#include <iomanip>
#include <iostream>
#include <istream>
#include <iterator>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <tuple>
#include <vector>
using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef vector<ll> vec;
typedef vector<vec> mat;
#define rep(i, n) for (ll i = 0; i < (n); i++)
#define revrep(i, n) for (ll i = (n)-1; i >= 0; i--)
#define pb push_back
#define f first
#define s second
ll max(ll a, ll b) { return (a > b) ? a : b; }
ll min(ll a, ll b) { return (a < b) ? a : b; }
ll max3(ll a, ll b, ll c) { return max(a, max(b, c)); };
ll min3(ll a, ll b, ll c) { return min(a, min(b, c)); };
ll max4(ll a, ll b, ll c, ll d) { return max(max(a, b), min(c, d)); };
ll min4(ll a, ll b, ll c, ll d) { return min(min(a, b), min(c, d)); };
ll max5(ll a, ll b, ll c, ll d, ll e) { return max(max(a, b), max3(c, d, e)); };
ll min5(ll a, ll b, ll c, ll d, ll e) { return min(min(a, b), min3(c, d, e)); };

const ll INFL = 1LL << 60; // 10^18 = 2^60
const int INF = 1 << 30;   // 10^9
ll MOD = 1000000007;
// ll MOD = 998244353;

vector<ll> dy = {0, 0, 1, -1, 1, 1, -1, -1, 0};
vector<ll> dx = {1, -1, 0, 0, 1, -1, 1, -1, 0};

ll pow_long(ll x, ll k) {
  ll res = 1;
  while (k > 0) {
    if (k % 2)
      res *= x;
    x *= x;
    k /= 2;
  }
  return res;
}
ll pow_mod(ll x, ll k) {
  x %= MOD;
  x += MOD;
  x %= MOD;
  ll res = 1;
  while (k > 0) {
    if (k % 2) {
      res *= x;
      res %= MOD;
    }
    x *= x;
    x %= MOD;
    k /= 2;
  }
  return res;
}

ll inverse(ll x) { return pow_mod(x, MOD - 2); };

ll gcd(ll a, ll b) {
  if (b == 0)
    return a;
  return gcd(b, a % b);
}

ll lcm(ll x, ll y) { return x / gcd(x, y) * y; };

ll kai_mod(ll x) {
  if (x == 0)
    return 1;
  return x * kai_mod(x - 1) % MOD;
}

/*
//コンビネーション
const int MAXcomb = 200010;
ll fac[MAXcomb], finv[MAXcomb], inv[MAXcomb];
//facはn!,finvは1/n!
//invは逆元
void COMinit(){
    fac[0] = fac[1] = 1;
    finv[0] = finv[1] = 1;
    inv[1] = 1;
    for(int i = 2; i < MAXcomb; i++){
        fac[i] = fac[i-1] * i % MOD;
        inv[i] = MOD - inv[MOD%i] * (MOD/i) % MOD;
        finv[i] = finv[i-1] * inv[i] % MOD;
    }
}
ll comb(int n, int k){
    if(n < k) return 0;
    if(n < 0 || k < 0) return 0;
    return fac[n] * finv[k] % MOD * finv[n-k] % MOD;
}
*/

ll N;
vector<ll> A(100010);
int main() {
  cin >> N;
  rep(i, N) cin >> A[i];
  bool l = 1;
  if (A[0])
    l = 0;
  rep(i, N - 1) if (A[i + 1] - A[i] > 1) l = 0;
  if (l == 0) {
    cout << -1 << endl;
    return 0;
  }

  ll ans = 0;
  ll at = N - 1;
  ll sta = -1;
  while (at >= 0) {
    if (sta != -1) {
      if (A[at] != sta) {
        ans += A[at];
        sta = A[at];
      }
    } else {
      ans += A[at];
      sta = A[at];
    }
    sta--;
    at--;
  }
  cout << ans << endl;
}


Fixed submission:
#include <algorithm>
#include <bitset>
#include <complex>
#include <cstring>
#include <deque>
#include <iomanip>
#include <iostream>
#include <istream>
#include <iterator>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <tuple>
#include <vector>
using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef vector<ll> vec;
typedef vector<vec> mat;
#define rep(i, n) for (ll i = 0; i < (n); i++)
#define revrep(i, n) for (ll i = (n)-1; i >= 0; i--)
#define pb push_back
#define f first
#define s second
ll max(ll a, ll b) { return (a > b) ? a : b; }
ll min(ll a, ll b) { return (a < b) ? a : b; }
ll max3(ll a, ll b, ll c) { return max(a, max(b, c)); };
ll min3(ll a, ll b, ll c) { return min(a, min(b, c)); };
ll max4(ll a, ll b, ll c, ll d) { return max(max(a, b), min(c, d)); };
ll min4(ll a, ll b, ll c, ll d) { return min(min(a, b), min(c, d)); };
ll max5(ll a, ll b, ll c, ll d, ll e) { return max(max(a, b), max3(c, d, e)); };
ll min5(ll a, ll b, ll c, ll d, ll e) { return min(min(a, b), min3(c, d, e)); };

const ll INFL = 1LL << 60; // 10^18 = 2^60
const int INF = 1 << 30;   // 10^9
ll MOD = 1000000007;
// ll MOD = 998244353;

vector<ll> dy = {0, 0, 1, -1, 1, 1, -1, -1, 0};
vector<ll> dx = {1, -1, 0, 0, 1, -1, 1, -1, 0};

ll pow_long(ll x, ll k) {
  ll res = 1;
  while (k > 0) {
    if (k % 2)
      res *= x;
    x *= x;
    k /= 2;
  }
  return res;
}
ll pow_mod(ll x, ll k) {
  x %= MOD;
  x += MOD;
  x %= MOD;
  ll res = 1;
  while (k > 0) {
    if (k % 2) {
      res *= x;
      res %= MOD;
    }
    x *= x;
    x %= MOD;
    k /= 2;
  }
  return res;
}

ll inverse(ll x) { return pow_mod(x, MOD - 2); };

ll gcd(ll a, ll b) {
  if (b == 0)
    return a;
  return gcd(b, a % b);
}

ll lcm(ll x, ll y) { return x / gcd(x, y) * y; };

ll kai_mod(ll x) {
  if (x == 0)
    return 1;
  return x * kai_mod(x - 1) % MOD;
}

/*
//コンビネーション
const int MAXcomb = 200010;
ll fac[MAXcomb], finv[MAXcomb], inv[MAXcomb];
//facはn!,finvは1/n!
//invは逆元
void COMinit(){
    fac[0] = fac[1] = 1;
    finv[0] = finv[1] = 1;
    inv[1] = 1;
    for(int i = 2; i < MAXcomb; i++){
        fac[i] = fac[i-1] * i % MOD;
        inv[i] = MOD - inv[MOD%i] * (MOD/i) % MOD;
        finv[i] = finv[i-1] * inv[i] % MOD;
    }
}
ll comb(int n, int k){
    if(n < k) return 0;
    if(n < 0 || k < 0) return 0;
    return fac[n] * finv[k] % MOD * finv[n-k] % MOD;
}
*/

ll N;
vector<ll> A(200010);
int main() {
  cin >> N;
  rep(i, N) cin >> A[i];
  bool l = 1;
  if (A[0])
    l = 0;
  rep(i, N - 1) if (A[i + 1] - A[i] > 1) l = 0;
  if (l == 0) {
    cout << -1 << endl;
    return 0;
  }

  ll ans = 0;
  ll at = N - 1;
  ll sta = -1;
  while (at >= 0) {
    if (sta != -1) {
      if (A[at] != sta) {
        ans += A[at];
        sta = A[at];
      }
    } else {
      ans += A[at];
      sta = A[at];
    }
    sta--;
    at--;
  }
  cout << ans << endl;
}