PAT 甲级 1144 The Missing Number (

/*
  PAT 甲级 1144 map
*/

#include <iostream>
#include <map>
using namespace std;
int main() {
    int n;
    map<int, int> m;
    scanf("%d", &n);
    for(int i = 0; i < n; i++){
        int v;
        scanf("%d", &v);
        m[v]++;
    }
    int i = 0;
    while(++i){
        if(m[i] == 0) {
            printf("%d\n", i);
            break;
        }
    }
    return 0;
}