A lake near the Arctic Circle is covered by a $2$-meter-thick sheet of ice during the cold winter months. When spring arrives, the warm air gradually melts the ice, causing its thickness to decrease at a constant rate. After $3$ weeks, the sheet is only $1.25$ meters thick. Let $S(t)$ denote the ice sheet's thickness $S$ (measured in meters) as a function of time $t$ (measured in weeks). **Write the function's formula.** $S(t)=$ [[☃ expression 1]]