Codeforces Testing Round #10 A. Forgotten Episode

水题，注意数据范围

#include <iostream>
using namespace std;int main(){long long n,a;cin >> n;long long  sum =(n*(n+1))>>1;for(int i = 0 ; i < n-1 ; ++i){cin >>a;sum -=a;}cout<<sum<<endl;
}