Why I have a Segmentation fault in this small program?

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

int main() {
    map<int, int> score;
    int n;
    cin >> n;
    while(n--){
        int a,b;
        cin >> a >> b;
        score[a] = score[a] + b;
    }
    cout << score.rbegin()->first << " " << score.rbegin()->second << endl;
    return 0;
}

the error message is 16254 segmentation fault::11. Exit code: 139. And could you please why segmentation fault frequently happened when using map container.