#include <stdlib.h>
#include <stdio.h>
#include <string.h>
int main (int argc, char ** argv) {
	size_t k[200000];
	char b[2000000];
	size_t d = 0;
	char * p = NULL;
	char ne[128];
	char * nu = fgets(ne, 127, stdin);
	nu++; nu--;
	size_t n = strtoll(ne, NULL, 10);
	char c = fgetc(stdin);
	while (!feof(stdin)) {
		b[d++] = c;
		c = fgetc(stdin);
	}
	if (b[d-1] == '\n') /* ce je input slucajno koncan z EOL */
		d--;
	if (b[d-1] == '\r') /* ce se slucajno uporablja slab operacijski sistem */
		d--;
	b[d] = '\0';
	d = 0;
	p = b;
	do {
		k[d] = strtol(p, &p, 10);
		p++; /* sizeof(char) je itak 1 */
		d++;
	} while (p[-1] != '\0');
	d;
	size_t korakov = 0;
	size_t temp = 0;
	for (long int zz = 0; zz < d; zz++) {
		/* fprintf(stderr, "%lu %lu %lu %lu %lu\n", k[0], k[1], k[2], k[3], k[4]);  */
		if (k[zz] == zz+1) {
			korakov++;
			temp = k[0];
			memcpy(&(k[0]), &(k[1]), sizeof(size_t)*(d-1));
			k[d-1] = temp;
			zz = -1;
		}
		if (korakov >= d) {
			fprintf(stdout, "-1\n");
			return 0;
		}
	}
	fprintf(stdout, "%lu\n", korakov);
	return 0;
}