luogu 1082

拓展歐幾里得的模板題（因爲他的b不一定是質數所以不能用費馬小定理來解決）
證明拓展歐幾里得的式子

p∗a+q∗b=GCD(a,b)=GCD(b,amodb)=p∗b+q∗(amodb)=

p∗b+q∗(a−(a/b)∗b)=q∗a+(p−(a/b)∗q)∗b

#include <cstdio>

using namespace std;

int a, b, x, y, d;

int read() {
    int f = 1, k = 0;
    char c = getchar();
    while(c < '0' || c > '9') {
        if( c == '-') {
            f = -1;
        }
        c = getchar();
    }
    while(c >= '0' && c <= '9') {
        k = k * 10 + c - '0';
        c = getchar();
    }
    return f * k;
}

int exgcd(int a, int b, int &d, int &x, int &y) {
    if(!b) {
        d = a;
        x = 1;
        y = 0;
    } else {
        exgcd(b, a % b, d, x, y) ;
        int x1 = x;
        x = y;
        y = x1 - (a / b) * y;
    }
}

int main() {
    a = read(), b = read();
    exgcd(a, b, d, x, y);
    printf("%d", (x + b) % b);
    return 0;
}