为啥不对

#include <bits/stdc++.h>
using namespace std;
int main() {
    int n;
    cin >> n;
    multimap<int, int> mp;
    for (int i=1;i<=n;i++) {
        int a,b;
        cin>>a>>b;
        mp.insert({b, a});
    }
    int ans = 0;
    int last = 0;
    for (auto &it : mp) {
        int a = it.first;
        int b = it.second;
        if (b >= last) {
            ans++;
            last=a;
        }
    }
    cout << ans;
    return 0;
}