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int gcd(int a,int b){
return b?gcd(b,a%b):a;
}void exgcd(int a,int b,int &d,int &x,int &y){
if(!b){x=1;y=0;d=a;}
else{exgcd(b,a%b,d,y,x);y-=a/b*x;}
}//r是（a/d）在模（n/d）意义下的逆
int linear_modular(int a,int b,int n){
int r,s,d;
exgcd(a,n,d,r,s);
if(b%d)return -1;
else return (r*(b/d)+n*abs(r))%(n/d);
}