C++ upper_bound is not working

Debugging Help
#1

I used the upper_bound function on a vector and this error occurred
no suitable conversion function from “__gnu_cxx::__normal_iterator<int *, std::vector<int, std::allocator>>” to “int” exists

Here is my code

#include <iostream>
#include <vector>
#include <algorithm>
#include <utility>
using namespace std;

int main(){
    ios::sync_with_stdio(false);
	cin.tie(nullptr);
    int k,m,n;
    cin>>k>>m>>n;
    vector<pair<int, int>> grass;
    vector<int> nhojcow;
    for(int i = 0; i < k; i++){
        int p,t;
        cin>>p>>t;
        grass.push_back({p,t});
    }
    for(int i = 0;i<m;i++) cin>>nhojcow[i];
    sort(begin(grass),end(grass));
    sort(begin(nhojcow), end(nhojcow));
    long long prefixsum[m+1];
    for(int i = 1;i<=m;i++) prefixsum[i] += prefixsum[i-1] + nhojcow[i-1];
    int index = 0;
    for(int i = 0; i<k;i++){
        int nextcow = upper_bound(begin(nhojcow), end(nhojcow), 1);
    } 
}

Thank everyone for helping me debug!

#2

NVM I figured it out.