#pragma GCC optimize("Ofast")
#pragma GCC target("avx2")

#define rd(v) long v=0;{long _c;while(_c=*rp++-48,_c>=0)v=v*10+_c;}
#define wt(v) {ulong _z=v;do*--wp=_z%10+48;while(_z/=10);}
#define rep(v,e) for(long v=0;v<e;++v)

typedef unsigned long ulong;
char*mmap();

int a[300000];

int main(){
	char*rp=mmap(0l,1l<<25,1,2,0,0ll);

	ulong h=0;
	ulong k=0;
	ulong m=1;

	rd(n);
	rep(i,n){
		rd(ai);
		a[i]=ai;
	}
	rep(i,n-1){
		if(a[i]>a[i+1]){
			h+=1;
		}
		h*=3;
		m*=3;
	}
	rep(i,n-1){
		if(rp[i]=='>'){
			k+=1;
		}
		k*=3;
	}
	long z=0;
	while(h!=k&&z<n){
		if(a[z]>a[z+1]){
			h-=m;
		}
		if(a[z?z-1:n-1]>a[z]){
			h+=1;
		}
		h*=3;
		++z;
	}

	char wbuf[64],*wp=wbuf+sizeof wbuf;
	if(z==n){
		*--wp='1';
		*--wp='-';
	}else{
		wt(z);
	}
	write(1,wp,wbuf+sizeof wbuf-wp);
	_exit(0);
}