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//接口：inv(a,n) a : 待求逆的数，n表示mod
//没有逆元，返回-1
void gcd(LL a,LL b,LL &d,LL &x,LL &y)
{
if(!b){d = a;x = 1;y = 0;}
else {gcd(b,a% b,d,y,x); y-= x * (a / b);}
}
LL inv(LL a,LL n)
{
LL d,x,y;
gcd(a,n,d,x,y);
return d == 1 ? (x + n) % n : -1;
}