Language: C

#include <math.h>
#define PI 3.14159265358979323

//int calc1(float a,float b,float c,float d,float e,float f,float *x,float *y);
//void quick(int *,long long,long long);
//int comp(char *a,char *b);

int main(){
	
	char a[100];
	char b[100];
	char c;
	int i,j,k,n;

	n=100;
	i=0;
	j=0;
	while(scanf("%c",&c)!=EOF){
		if(c=='\n'){
			j--;
			if(j==2&&( (b[0]-'t')-(b[1]-'h')+52 )%26==0&&( (b[0]-'t')-(b[2]-'e')+52 )%26==0)
				n=b[0]-'t';
			if(j==3&&( (b[0]-'t')-(b[1]-'h')+52 )%26==0&&( (b[0]-'t')-(b[2]-'i')+52 )%26==0&&( (b[0]-'t')-(b[3]-'s')+52 )%26==0)
				n=b[0]-'t';
			if(j==3&&( (b[0]-'t')-(b[1]-'h')+52 )%26==0&&( (b[0]-'t')-(b[2]-'a')+52 )%26==0&&( (b[0]-'t')-(b[3]-'t')+52 )%26==0)
				n=b[0]-'t';
			n+=52;
			n=n%26;

			if(n==100)
				printf("error");
			for(j=0;j<i;j++){
				if(a[j]>='a'&&a[j]<='z'){
					a[j]-=n;
					if(a[j]<'a')
						a[j]+=26;
					else if(a[j]>'z')
						a[j]-=26;
				}
					printf("%c",a[j]);
			}
			printf("\n");
			n=100;
			i=0;
			j=0;
		}else{
			a[i]=c;
			i++;
			if(c>='a'&&c<='z'){
				b[j]=c;
				j++;
				if(j>10)
					j=0;
			}else{
				j--;
				if(j==2&&( (b[0]-'t')-(b[1]-'h')+52 )%26==0&&( (b[0]-'t')-(b[2]-'e')+52 )%26==0)
					n=b[0]-'t';
				if(j==3&&( (b[0]-'t')-(b[1]-'h')+52 )%26==0&&( (b[0]-'t')-(b[2]-'i')+52 )%26==0&&( (b[0]-'t')-(b[3]-'s')+52 )%26==0)
					n=b[0]-'t';
				if(j==3&&( (b[0]-'t')-(b[1]-'h')+52 )%26==0&&( (b[0]-'t')-(b[2]-'a')+52 )%26==0&&( (b[0]-'t')-(b[3]-'t')+52 )%26==0)
					n=b[0]-'t';
				j=0;
			}
		}
	}
	//for(i=0;i<=1000000000;i++);
	return 0;
}

Compiler output:
main.c: In function 'main':
main.c:18:15: error: implicit declaration of function 'scanf' [-Wimplicit-function-declaration]
   18 |         while(scanf("%c",&c)!=EOF){
      |               ^~~~~
main.c:2:1: note: include '<stdio.h>' or provide a declaration of 'scanf'
    1 | #include <math.h>
  +++ |+#include <stdio.h>
    2 | #define PI 3.14159265358979323
main.c:18:15: warning: incompatible implicit declaration of built-in function 'scanf' [-Wbuiltin-declaration-mismatch]
   18 |         while(scanf("%c",&c)!=EOF){
      |               ^~~~~
main.c:18:15: note: include '<stdio.h>' or provide a declaration of 'scanf'
main.c:18:31: error: 'EOF' undeclared (first use in this function)
   18 |         while(scanf("%c",&c)!=EOF){
      |                               ^~~
main.c:18:31: note: 'EOF' is defined in header '<stdio.h>'; this is probably fixable by adding '#include <stdio.h>'
main.c:18:31: note: each undeclared identifier is reported only once for each function it appears in
main.c:31:33: error: implicit declaration of function 'printf' [-Wimplicit-function-declaration]
   31 |                                 printf("error");
      |                                 ^~~~~~
main.c:31:33: note: include '<stdio.h>' or provide a declaration of 'printf'
main.c:31:33: warning: incompatible implicit declaration of built-in function 'printf' [-Wbuiltin-declaration-mismatch]
main.c:31:33: note: include '<stdio.h>' or provide a declaration of 'printf'
main.c:40:41: warning: incompatible implicit declaration of built-in function 'printf' [-Wbuiltin-declaration-mismatch]
   40 |                                         printf("%c",a[j]);
      |                                         ^~~~~~
main.c:40:41: note: include '<stdio.h>' or provide a declaration of 'printf'
main.c:42:25: warning: incompatible implicit declaration of built-in function 'printf' [-Wbuiltin-declaration-mismatch]
   42 |                         printf("\n");
      |                         ^~~~~~
main.c:42:25: note: include '<stdio.h>' or provide a declaration of 'printf'