#include<stdio.h>
#include<stdlib.h>
int main(int argc,char*argv[])
{
    if(argc<2)
    {
        printf("Invalid input\n");
        return0;
    }
    int n=atoi(argv[1]);
    if(argc!=n+1||n<2)
    {
        printf("Invalid input\n");
        return 0;
    }
    int sum=0;
    for(int i=2;i<argc;i++)
    {
        sum+=atoi(argv[i]);
    }
    int total=n*(n+1)/2;
    int missing=total-sum;
    prinf("%d\n",missing);
}