#include <iostream>
#include <cstdio>
#include <vector>
#include <iterator>
#include <algorithm>
#include <utility>

int main()
{
    std::ios_base::sync_with_stdio(false);
    
    int N;
    std::vector<int> L;
    L.reserve(1000000);

    while (std::cin >> N) {
        // input
        L.clear();
        L.push_back(0);
        L.push_back(0);
        for (int i = 0; i < N; ++i) {
            L.push_back(i);
            std::cin >> L.back();
        }
        
        if (N < 4) {
            std::cout << "OK\n";
            continue;
        }
        
        std::vector<int>::iterator l = L.begin() + 4;
        
        // unwind
        for (; l != L.end(); ++l) {
            if (*l <= *(l - 2)) {
                int gap = *(l - 2) - *(l - 4);
                if (*l >= gap) {
                    *(l - 1) = *(l - 1) - *(l - 3);
                }
                //std::cout << "unwind done at " << (l - L.begin() - 1) << "\n";
                break;
            }
        }
        
        
        ++l;
        
        // wind
        bool fail = false;
        for (; l < L.end(); ++l) {
            if (*l >= *(l - 2)) {
                fail = true;
                break;
            }
        }
        
        // output
        if (fail) {
            std::cout << (l - L.begin() - 2) << '\n';
        } else {
            std::cout << "OK\n";
        }
    }
    
    return 0;
}