#include <bits/stdc++.h>

int main()
{
    int buffer[1000 + 1];
    int i, N;
    long long int diff;
    
    while ( scanf("%d", &N) == 1 )
    {
        memset(buffer,0,sizeof(buffer));
        for ( i = 1; i <= N; i++)
        {
            scanf("%d", &buffer[i]);
        }
        if ( N < 3 ) // trivial case
        {
            printf("1\n");
            continue;
        }
        diff = buffer[N] - buffer[N-1];
        for ( i = N - 1; i > 1; i-- )
        {
            if ( (buffer[i] - buffer[i-1]) != diff )
                break;
        }
    printf("%d\n",i);
    
    }




return 0;
}