C

#include <cstdio>

const int NMAX = 100005;
int a[NMAX], n, tmax, tcnt;

int main() {
    scanf("%d", &n);
    for(int i = 0; i < n; i++) scanf("%d", &a[n - i - 1]);
    while (n > 0) {
        int j = n - 1, cnt;
        while (j > 0 && a[n - 1] == a[j - 1]) --j;
        cnt = n - j;
        if (cnt % 2 == 1) {
            tcnt++, n--;
            if (a[n] > tmax) tmax = a[n];
        }
        cnt /= 2;
        n -= cnt;
        for(int i = 0; i < cnt; i++) a[n - 1 - i]++;
    }
    printf("%d\n", tmax - tcnt + 1);
}