//#define NDEBUG

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>

#include <assert.h>
// for DEBUG {{{
#ifdef NDEBUG
#define DEBUG_PRINT(fmt, ...) ((void)0)
#else
#define DEBUG_PRINT(fmt, ...) fprintf(stderr, fmt, ##__VA_ARGS__)
#endif
#define ASSERT_RANGE(min,x,max) assert(((x)>=(min)) && ((x)<=(max)))
// }}}

typedef long long ll;

#define GET_D(x) fscanf(stdin,"%d",&(x)) > 0
#define GET_LLD(x) fscanf(stdin,"%lld",&(x)) > 0
#define GET_S(x) fscanf(stdin,"%s",(x)) > 0

#define PUT_S(x) fprintf(stdout,"%s\n",(x))
#define PUT_D(x) fprintf(stdout,"%d\n",(x))
#define PUT_Ds(x) fprintf(stdout,"%d ",(x))
#define PUT_LLD(x) fprintf(stdout,"%lld\n",(x))
#define PUT_LLDs(x) fprintf(stdout,"%lld ",(x))
#define PUT_LN fprintf(stdout,"\n")

#define FOR(i,min,max) for ((i) = (min); (i) < (max); (i)++)

void merge_int(int *val, int lower, int sep, int upper, int *tmp_val) {// {{{
	int i_lower = lower;
	int i_upper = sep;
	int i_b = 0;
	while ((i_lower < sep) && (i_upper < upper)) {
		if (val[i_lower] < val[i_upper]) {
			tmp_val[i_b++] = val[i_lower++];
		} else {
			tmp_val[i_b++] = val[i_upper++];
		}
	}
	if (i_lower >= sep) {
		while (i_upper < upper) {
			tmp_val[i_b++] = val[i_upper++];
		}
	} else {
		while (i_lower < sep) {
			tmp_val[i_b++] = val[i_lower++];
		}
	}
	for (i_lower = 0; i_lower < i_b; i_lower++) {
		val[i_lower+lower] = tmp_val[i_lower];
	}
}// }}}
void merge_sort_int(int *val, int lower, int upper, int *tmp_val) {// {{{
	int sep;
	if ((lower == upper) || (lower + 1 == upper)) {
		return;
	}
	sep = (lower + upper) / 2;
	merge_sort_int(val,lower,sep,tmp_val);
	merge_sort_int(val,sep,upper,tmp_val);
	merge_int(val,lower,sep,upper,tmp_val);
	return;
}// }}}

void merge_int_int(int *val, int *index, int lower, int sep, int upper, int *tmp_val, int *tmp_index) {// {{{
	int i_lower = lower;
	int i_upper = sep;
	int i_b = 0;
	while ((i_lower < sep) && (i_upper < upper)) {
		if (val[i_lower] < val[i_upper]) {
			tmp_val[i_b] = val[i_lower];
			tmp_index[i_b] = index[i_lower];
			i_b++;
			i_lower++;
		} else {
			tmp_val[i_b] = val[i_upper];
			tmp_index[i_b] = index[i_upper];
			i_b++;
			i_upper++;
		}
	}
	if (i_lower >= sep) {
		while (i_upper < upper) {
			tmp_val[i_b] = val[i_upper];
			tmp_index[i_b] = index[i_upper];
			i_b++;
			i_upper++;
		}
	} else {
		while (i_lower < sep) {
			tmp_val[i_b] = val[i_lower];
			tmp_index[i_b] = index[i_lower];
			i_b++;
			i_lower++;
		}
	}
	for (i_lower = 0; i_lower < i_b; i_lower++) {
		val[i_lower+lower] = tmp_val[i_lower];
		index[i_lower+lower] = tmp_index[i_lower];
	}
}// }}}
void merge_sort_int_int(int *val, int *index, int lower, int upper, int *tmp_val, int *tmp_index) {// {{{
	int sep;
	if ((lower == upper) || (lower + 1 == upper)) {
		return;
	}
	sep = (lower + upper) / 2;
	merge_sort_int_int(val,index,lower,sep,tmp_val,tmp_index);
	merge_sort_int_int(val,index,sep,upper,tmp_val,tmp_index);
	merge_int_int(val,index,lower,sep,upper,tmp_val,tmp_index);
	return;
}// }}}

void merge_ll(ll *val, ll lower, ll sep, ll upper, ll *tmp_val) {// {{{
	ll i_lower = lower;
	ll i_upper = sep;
	ll i_b = 0;
	while ((i_lower < sep) && (i_upper < upper)) {
		if (val[i_lower] < val[i_upper]) {
			tmp_val[i_b++] = val[i_lower++];
		} else {
			tmp_val[i_b++] = val[i_upper++];
		}
	}
	if (i_lower >= sep) {
		while (i_upper < upper) {
			tmp_val[i_b++] = val[i_upper++];
		}
	} else {
		while (i_lower < sep) {
			tmp_val[i_b++] = val[i_lower++];
		}
	}
	for (i_lower = 0; i_lower < i_b; i_lower++) {
		val[i_lower+lower] = tmp_val[i_lower];
	}
}// }}}
void merge_sort_ll(ll *val, ll lower, ll upper, ll *tmp_val) {// {{{
	ll sep;
	if ((lower == upper) || (lower + 1 == upper)) {
		return;
	}
	sep = (lower + upper) / 2;
	merge_sort_ll(val,lower,sep,tmp_val);
	merge_sort_ll(val,sep,upper,tmp_val);
	merge_ll(val,lower,sep,upper,tmp_val);
	return;
}// }}}

void merge_ll_ll(ll *val, ll *index, ll lower, ll sep, ll upper, ll *tmp_val, ll *tmp_index) {// {{{
	ll i_lower = lower;
	ll i_upper = sep;
	ll i_b = 0;
	while ((i_lower < sep) && (i_upper < upper)) {
		if (val[i_lower] < val[i_upper]) {
			tmp_val[i_b] = val[i_lower];
			tmp_index[i_b] = index[i_lower];
			i_b++;
			i_lower++;
		} else {
			tmp_val[i_b] = val[i_upper];
			tmp_index[i_b] = index[i_upper];
			i_b++;
			i_upper++;
		}
	}
	if (i_lower >= sep) {
		while (i_upper < upper) {
			tmp_val[i_b] = val[i_upper];
			tmp_index[i_b] = index[i_upper];
			i_b++;
			i_upper++;
		}
	} else {
		while (i_lower < sep) {
			tmp_val[i_b] = val[i_lower];
			tmp_index[i_b] = index[i_lower];
			i_b++;
			i_lower++;
		}
	}
	for (i_lower = 0; i_lower < i_b; i_lower++) {
		val[i_lower+lower] = tmp_val[i_lower];
		index[i_lower+lower] = tmp_index[i_lower];
	}
}// }}}
void merge_sort_ll_ll(ll *val, ll *index, ll lower, ll upper, ll *tmp_val, ll *tmp_index) {// {{{
	ll sep;
	if ((lower == upper) || (lower + 1 == upper)) {
		return;
	}
	sep = (lower + upper) / 2;
	merge_sort_ll_ll(val,index,lower,sep,tmp_val,tmp_index);
	merge_sort_ll_ll(val,index,sep,upper,tmp_val,tmp_index);
	merge_ll_ll(val,index,lower,sep,upper,tmp_val,tmp_index);
	return;
}// }}}


#define N_MIN 1
#define N_MAX 200000

int main(int argc, char **argv) {
	int n;
	int a;

	GET_D(n);
	{
		int i;
		int j = 1;
		FOR(i,0,n) {
			GET_D(a);
			if (a == j) {
				j++;
			}
		}
		if (j == 1) {
			PUT_D(-1);
		} else {
			PUT_D(n-(j-1));
		}
	}
	return 0;
}



Measured memory_bytes: 128