I have found the correct solution for this problem(407B - Long Path), but I'm having problems with the modulo. It's behaving strangely: it gives me negative value. Do you have a guess of why that? Here's my code and my submission(10170884).

#include <stdio.h>
#include <math.h>
#include <iostream>
#include <string.h>


#define INF 1000000005
#define MAX_N 3000005
#define MOD int(1e9)+7

using namespace std;

long long memo[MAX_N], p[MAX_N], n;

int main(){
    ios_base::sync_with_stdio(false);

    cin>>n;

    for(int i=0;i<n;i++){
        cin>>p[i];
        p[i]--;
    }

    for(int i=1;i<=n;i++){
        memo[i] = 2*memo[i-1] - memo[p[i-1]] + 2;
        memo[i] %= MOD;
    }
    cout<<memo[n];
    return 0;
}