pku 2115 C Looooops 同余定理

 #include <stdio.h>
long long mod(long long a,long long b)
{
return (a % b + b) % b;
}

struct triple
{
long long d,x,y;
};

long long Euclid(long long a,long long b)
{
if(b == 0)
return a;
else
return Euclid(b,mod(a,b));
}

triple Extended_Euclid(long long a,long long b)
{
triple result;
if(b == 0)
{
result.d = a;
result.x = 1;
result.y = 0;
}
else
{
triple ee = Extended_Euclid(b,mod(a,b));
result.d = ee.d;
result.x = ee.y;
result.y = ee.x - (a/b)*ee.y;
}
return result;
}
long long MLES(long long a,long long b,long long n)
{
triple ee = Extended_Euclid(a,n);
if(mod(b,ee.d) == 0)
return mod((ee.x * (b / ee.d)),n/ee.d);
else
return -1;
}
int main()
{
long long A, B, C, k;
while(scanf("%I64d%I64d%I64d%I64d", &A, &B, &C, &k) && !(A==0&&B==0&&C==0&&k==0))
{
if(A == B)
{
printf("0\n");
continue;
}
long long ret = MLES(C, B-A, ((long long)1)<<k);
//((long long)1)<<k
//这儿要注意,1<<32位后,会超过int的最大值,需要转换为long long
if(ret == -1)
printf("FOREVER\n");
else
printf("%I64d\n", ret);
}
return 0;
}