my code outputs is exactly as the test cases only on codeBlockes and other online compiler, but in CF it is different, i think it is undefined behavior but i cannot spot it in the code.

#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

int main()
{
    int n,m,a,b,cnt;
    vector<int>arr;
    cin>>n>>m;

    for(int i=0;i<m;i++)
    {
        cin>>a>>b;
       arr.insert(arr.end(), a, b);

    }

  sort(arr.rbegin(), arr.rend());




    for(int i=0;i<n;i++)
    {
        cnt+=arr[i];

    }

    cout<<cnt;

    return 0;
}