How Much Home Equity Do We Have If The Home We Purchased Was In Foreclosure

I know this sounds crazy to im having trouble home equity Understand When It Comes to a foreclosure. We Purchased a home about 4 years ago That Was in foreclosure for about 50k more Than The original price at The Time That WAS the-home first built. Is the home equity just based On the current market value minus What We Have Paid For the previous 4 years Does The home equity gold date back to home When The first WAS Purchased By The previous owners? Thanks.