tree.cpp

#include <cstdio>
#include <algorithm>
#include <vector>

int n;
std::vector<int> a;

int main(){
    int i, tmp;
    int count_1 = 0, count_2 = 0, len = 0;

    scanf("%d", &n);

    if (n <= 0){
        printf("0");
        return 0;
    }

    if (n == 1){
        printf("1");
        return 0;
    }

    for (i = 0; i < n; i++){
        scanf("%d", &(tmp));
        if (tmp == 1){
            count_1++;
        } else if (tmp == 2){
            count_2++;
        } else {
            a.push_back(tmp);
        }
    }

    std::sort(a.begin(), a.end(), std::greater<int>());

    if (count_1 < 2){
        printf("-1");
        return 0;
    }

    count_1 -= 1;
    len = 1 + count_2;

    while (!a.empty()){
        int curr = a.back();
        if ((count_1 == (curr-1)) && (a.size() == 1)){
            len++;
            break;
        } else if ((count_1 != (curr-1)) && (a.size() == 1)){
            printf("-1");
            return 0;
        } else if (count_1 < (curr - 2)){
            printf("-1");
            return 0;
        } else {
            count_1 -= (curr - 2);
            len += 1;
            a.pop_back();
        }
    }

    printf("%d", len);
}