#include<bits/stdc++.h>
using namespace std;
 
int trips(vector<int> &weights){
 
    int ans = 0;
    map<int, int> freq;
 
    for(auto val : weights){
        freq[val]++;
    }
 
    for(auto obj : freq){
    	int val = obj.second;
 
        if(val == 1){
        	return -1;
        }
 
        ans += val/3;
        if(val%3 != 0){
        	ans += 1;
        }
    }
 
    return ans;
}
 
int main(){
    vector<int> weights = {2, 4, 6, 6, 4, 2, 4};
    cout << trips(weights);
    return 0;
}

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