Managed C++/CLI

// swap here as already done
// Can use the same code as the encoding because there is no difference in the result

// Reverse adding 7
iFirst -= 7;
// ...

// Reverse modulo 10 operation
if (iFirst < 0)
    iFirst += 10;
// ...

#include<stdio.h>
void main(){
int i[4];
printf("\nEnter four digits (with a space in between): ");
for(int j=0;j<4;j++)
{
scanf("%d", &i[j]);
}
int k=0;
k = i[0];
i[0] = i[2];
i[2] = k;
k = i[1];
i[1] = i[3];
i[3] = k;
for(int j=0;j<4;j++)
{
if(i[j]<7)
{
i[j]=i[j]+3;
}
else
{
i[j]=i[j]-7;
}
printf("%d",i[j]);
}
}

"This is line 1\r\n"
"This is line 2\r\n"
etc.