P6.14 P89851 Which is missing?

#include <iostream>
using namespace std;

int main() {
    int n,x;
    while (cin>>n) {
        int totalx=0,totaln=n;
        for (int i=n-1;i>0;i--) {
            cin>>x;
            totalx=totalx+x;
            totaln=totaln+i;
        }
        cout<<totaln-totalx<<endl;
    }
}